PCI: hv: Add explicit barriers to config space access

I'm trying to pass-through Broadcom BCM5720 NIC (Dell device 1f5b) on a
Dell R720 server.  Everything works fine when the target VM has only one
CPU, but SMP guests reboot when the NIC driver accesses PCI config space
with hv_pcifront_read_config()/hv_pcifront_write_config().  The reboot
appears to be induced by the hypervisor and no crash is observed.  Windows
event logs are not helpful at all ('Virtual machine ... has quit
unexpectedly').  The particular access point is always different and
putting debug between them (printk/mdelay/...) moves the issue further
away.  The server model affects the issue as well: on Dell R420 I'm able to
pass-through BCM5720 NIC to SMP guests without issues.

While I'm obviously failing to reveal the essence of the issue I was able
to come up with a (possible) solution: if explicit barriers are added to
hv_pcifront_read_config()/hv_pcifront_write_config() the issue goes away.
The essential minimum is rmb() at the end on _hv_pcifront_read_config() and
wmb() at the end of _hv_pcifront_write_config() but I'm not confident it
will be sufficient for all hardware.  I suggest the following barriers:

1) wmb()/mb() between choosing the function and writing to its space.
2) mb() before releasing the spinlock in both _hv_pcifront_read_config()/
   _hv_pcifront_write_config() to ensure that consecutive reads/writes to
  the space won't get re-ordered as drivers may count on that.

Config space access is not supposed to be performance-critical so these
explicit barriers should not cause any slowdown.

[bhelgaas: use Linux "barriers" terminology]
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jake Oshins <jakeo@microsoft.com>

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