pkg/fileutils: GetTotalUsedFds(): don't pretend to support FreeBSD

Commit 8d56108ffb 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>

pkg/fileutils/fileutils_linux.go

@@ -1,6 +1,4 @@
-//go:build linux || freebsd
-
-package fileutils // import "github.com/docker/docker/pkg/fileutils"
+package fileutils
 
 import (
 	"context"