VFS: Make delayed_free() call free_vfsmnt()

Make delayed_free() call free_vfsmnt() so that we don't have two functions
doing the same job.  This requires the calls to mnt_free_id() in free_vfsmnt()
to be moved into the callers of that function.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>