proc: report open files as size in stat() for /proc/pid/fd

Many monitoring tools include open file count as a metric.  Currently the
only way to get this number is to enumerate the files in /proc/pid/fd.

The problem with the current approach is that it does many things people
generally don't care about when they need one number for a metric.  In our
tests for cadvisor, which reports open file counts per cgroup, we observed
that reading the number of open files is slow.  Out of 35.23% of CPU time
spent in `proc_readfd_common`, we see 29.43% spent in `proc_fill_cache`,
which is responsible for filling dentry info.  Some of this extra time is
spinlock contention, but it's a contention for the lock we don't want to
take to begin with.

We considered putting the number of open files in /proc/pid/status. 
Unfortunately, counting the number of fds involves iterating the
open_files bitmap, which has a linear complexity in proportion with the
number of open files (bitmap slots really, but it's close).  We don't want
to make /proc/pid/status any slower, so instead we put this info in
/proc/pid/fd as a size member of the stat syscall result.  Previously the
reported number was zero, so there's very little risk of breaking
anything, while still providing a somewhat logical way to count the open
files with a fallback if it's zero.

RFC for this patch included iterating open fds under RCU.  Thanks to Frank
Hofmann for the suggestion to use the bitmap instead.

Previously:

```
$ sudo stat /proc/1/fd | head -n2
  File: /proc/1/fd
  Size: 0         	Blocks: 0          IO Block: 1024   directory
```

With this patch:

```
$ sudo stat /proc/1/fd | head -n2
  File: /proc/1/fd
  Size: 65        	Blocks: 0          IO Block: 1024   directory
```

Correctness check:

```
$ sudo ls /proc/1/fd | wc -l
65
```

I added the docs for /proc/<pid>/fd while I'm at it.

[ivan@cloudflare.com: use bitmap_weight() to count the bits]
  Link: https://lkml.kernel.org/r/20221018045844.37697-1-ivan@cloudflare.com
[akpm@linux-foundation.org: include linux/bitmap.h for bitmap_weight()]
[ivan@cloudflare.com: return errno from proc_fd_getattr() instead of setting negative size]
  Link: https://lkml.kernel.org/r/20221024173140.30673-1-ivan@cloudflare.com
Link: https://lkml.kernel.org/r/20220922224027.59266-1-ivan@cloudflare.com
Signed-off-by: Ivan Babrou <ivan@cloudflare.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Ivan Babrou <ivan@cloudflare.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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