tree-wide: make sure net/if.h is included before any linux/ header

The linux/ headers include linux/libc-compat.h that makes sure the
linux/ headers won't redeclare symbols already declared by net/if.h, but
glibc's net/if.h doesn't do that, so if the include order is reversed
we'll end up with a bunch of errors about redeclared stuff:

[3/519] Compiling C object test-network-tables.p/src_network_test-network-tables.c.o
FAILED: test-network-tables.p/src_network_test-network-tables.c.o
cc -Itest-network-tables.p -I. -I.. -Isrc/basic -I../src/basic -Isrc/fundamental -I../src/fundamental -Isrc/systemd -I../src/systemd -I../src/libsystemd/sd-bus -I../src/libsystemd/sd-device -I../src/libsystemd/sd-event -I../src/libsystemd/sd-hwdb -I../src/libsystemd/sd-id128 -I../src/libsystemd/sd-journal -I../src/libsystemd/sd-netlink -I../src/libsystemd/sd-network -I../src/libsystemd/sd-resolve -Isrc/shared -I../src/shared -Isrc/libsystemd-network -I../src/libsystemd-network -Isrc/network -I../src/network -I../src/network/netdev -I../src/network/tc -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wextra -std=gnu11 -O0 -g -Wno-missing-field-initializers -Wno-unused-parameter -Wno-nonnull-compare -Warray-bounds -Warray-bounds=2 -Wdate-time -Wendif-labels -Werror=format=2 -Werror=format-signedness -Werror=implicit-function-declaration -Werror=implicit-int -Werror=incompatible-pointer-types -Werror=int-conversion -Werror=missing-declarations -Werror=missing-prototypes -Werror=overflow -Werror=override-init -Werror=return-type -Werror=shift-count-overflow -Werror=shift-overflow=2 -Werror=strict-flex-arrays -Werror=undef -Wfloat-equal -Wimplicit-fallthrough=5 -Winit-self -Wlogical-op -Wmissing-include-dirs -Wmissing-noreturn -Wnested-externs -Wold-style-definition -Wpointer-arith -Wredundant-decls -Wshadow -Wstrict-aliasing=2 -Wstrict-prototypes -Wsuggest-attribute=noreturn -Wunused-function -Wwrite-strings -Wzero-length-bounds -fdiagnostics-show-option -fno-common -fstack-protector -fstack-protector-strong -fstrict-flex-arrays --param=ssp-buffer-size=4 -Wno-unused-result -Werror=shadow -fno-strict-aliasing -fstrict-flex-arrays=1 -fvisibility=hidden -fno-omit-frame-pointer -include config.h -pthread -DTEST_CODE=1 -MD -MQ test-network-tables.p/src_network_test-network-tables.c.o -MF test-network-tables.p/src_network_test-network-tables.c.o.d -o test-network-tables.p/src_network_test-network-tables.c.o -c ../src/network/test-network-tables.c
In file included from ../src/basic/linux/if_bonding.h:47,
                 from ../src/network/netdev/bond.h:5,
                 from ../src/network/test-network-tables.c:3:
../src/basic/linux/if.h:111:41: error: redeclaration of enumerator ‘IFF_UP’
  111 | #define IFF_UP                          IFF_UP
      |                                         ^~~~~~
../src/basic/linux/if.h:84:9: note: previous definition of ‘IFF_UP’ with type ‘enum net_device_flags’
   84 |         IFF_UP                          = 1<<0,  /* sysfs */
      |         ^~~~~~
../src/basic/linux/if.h:112:41: error: redeclaration of enumerator ‘IFF_BROADCAST’
  112 | #define IFF_BROADCAST                   IFF_BROADCAST
      |                                         ^~~~~~~~~~~~~
...

This also drops remaining workarounds from the last time this issue was
brought up (6f270e6bd8) since they shouldn't be needed anymore if the
order of the includes is the "correct" one. I also added a comment to
each affected include when this is inevitably encountered again in the
future.

Resolves: #32160

2 files changed, 4 insertions, 10 deletions

diff --git a/src/shared/firewall-util-iptables.c b/src/shared/firewall-util-iptables.c
index b70b74074b..044d2d0744 100644
--- a/src/shared/firewall-util-iptables.c
+++ b/src/shared/firewall-util-iptables.c
@@ -1,19 +1,12 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
-/* Temporary work-around for broken glibc vs. linux kernel header definitions
- * This is already fixed upstream, remove this when distributions have updated.
- */
-#define _NET_IF_H 1
-
+/* Make sure the net/if.h header is included before any linux/ one */
+#include <net/if.h>
 #include <arpa/inet.h>
 #include <endian.h>
 #include <errno.h>
 #include <stddef.h>
 #include <string.h>
-#include <net/if.h>
-#ifndef IFNAMSIZ
-#define IFNAMSIZ 16
-#endif
 #include <linux/if.h>
 #include <linux/netfilter_ipv4/ip_tables.h>
 #include <linux/netfilter/nf_nat.h>
diff --git a/src/shared/socket-netlink.c b/src/shared/socket-netlink.c
index 6865b045b5..a126d5d73b 100644
--- a/src/shared/socket-netlink.c
+++ b/src/shared/socket-netlink.c
@@ -1,9 +1,10 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+/* Make sure the net/if.h header is included before any linux/ one */
+#include <net/if.h>
 #include <arpa/inet.h>
 #include <errno.h>
 #include <linux/net_namespace.h>
-#include <net/if.h>
 #include <string.h>
 
 #include "alloc-util.h"