Commit 8d56108 moved this function from
the generic (no build-tags) fileutils.go to a unix file, adding "freebsd"
to the build-tags.

This likely was a wrong assumption (as other files had freebsd build-tags).
FreeBSD's procfs does not mention `/proc/<pid>/fd` in the manpage, and
we don't test FreeBSD in CI, so let's drop it, and make this a Linux-only
file.

While updating also dropping the import-tag, as we're planning to move
this file internal to the daemon.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 252e94f)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

    go test -bench ^BenchmarkGetTotalUsedFds$ -run ^$ ./pkg/fileutils/
    goos: linux
    goarch: arm64
    pkg: github.com/docker/docker/pkg/fileutils
    BenchmarkGetTotalUsedFds-5   	  149272	      7896 ns/op	     945 B/op	      20 allocs/op

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 03390be)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Use File.Readdirnames instead of os.ReadDir, as we're only interested in
the number of files, and results don't have to be sorted.

Before:

    BenchmarkGetTotalUsedFds-5   	  149272	      7896 ns/op	     945 B/op	      20 allocs/op

After:

    BenchmarkGetTotalUsedFds-5   	  153517	      7644 ns/op	     408 B/op	      10 allocs/op

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit eaa9494)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Linux 6.2 and up (commit [f1f1f2569901ec5b9d425f2e91c09a0e320768f3][1])
provides a fast path for the number of open files for the process.

From the [Linux docs][2]:

> The number of open files for the process is stored in 'size' member of
> `stat()` output for /proc/<pid>/fd for fast access.

[1]: torvalds/linux@f1f1f25
[2]: https://docs.kernel.org/filesystems/proc.html#proc-pid-fd-list-of-symlinks-to-open-files

This patch adds a fast-path for Kernels that support this, and falls back
to the slow path if the Size fields is zero.

Comparing on a Fedora 38 (kernel 6.2.9-300.fc38.x86_64):

Before/After:

    go test -bench ^BenchmarkGetTotalUsedFds$ -run ^$ ./pkg/fileutils/
    BenchmarkGetTotalUsedFds        57264     18595 ns/op     408 B/op      10 allocs/op
    BenchmarkGetTotalUsedFds       370392      3271 ns/op      40 B/op       3 allocs/op

Note that the slow path has 1 more file-descriptor, due to the open
file-handle for /proc/<pid>/fd during the calculation.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit ec79d0f)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>