ceph: ensure we have Fs caps when fetching dir link count

The link count for a directory is defined as inode->i_subdirs + 2,
(for "." and ".."). i_subdirs is only populated when Fs caps are held.
Ensure we grab Fs caps when fetching the link count for a directory.

[ idryomov: break unnecessarily long line ]

URL: https://tracker.ceph.com/issues/48125


Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>