ceph: add missing check in d_revalidate snapdir handling

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

Cc: stable@vger.kernel.org
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>

fs/ceph/inode.c

@@ -1434,6 +1434,7 @@ int ceph_fill_trace(struct super_block *sb, struct ceph_mds_request *req)
 		dout(" final dn %p\n", dn);
 	} else if ((req->r_op == CEPH_MDS_OP_LOOKUPSNAP ||
 		    req->r_op == CEPH_MDS_OP_MKSNAP) &&
+	           test_bit(CEPH_MDS_R_PARENT_LOCKED, &req->r_req_flags) &&
 		   !test_bit(CEPH_MDS_R_ABORTED, &req->r_req_flags)) {
 		struct inode *dir = req->r_parent;