ceph: add missing check in d_revalidate snapdir handling

commit 1f08529c upstream.

We should not play with dcache without parent locked...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
[bwh: Backported to 3.16:
 - Test ceph_mds_request::r_locked_dir
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>