ceph: tolerate (and warn on) extraneous dentry from mds

If the MDS gives us a dentry and we weren't prepared to handle it,
WARN_ON_ONCE instead of crashing.
Reported-by: NYan, Zheng <zheng.z.yan@intel.com>
Signed-off-by: NSage Weil <sage@inktank.com>
Reviewed-by: NAlex Elder <elder@inktank.com>

fs/ceph/inode.c

@@ -992,11 +992,15 @@ int ceph_fill_trace(struct super_block *sb, struct ceph_mds_request *req,
 	if (rinfo->head->is_dentry) {
 		struct inode *dir = req->r_locked_dir;
 
+		if (dir) {
 			err = fill_inode(dir, &rinfo->diri, rinfo->dirfrag,
 					 session, req->r_request_started, -1,
 					 &req->r_caps_reservation);
 			if (err < 0)
 				return err;
+		} else {
+			WARN_ON_ONCE(1);
+		}
 	}
 
 	/*
@@ -1004,6 +1008,7 @@ int ceph_fill_trace(struct super_block *sb, struct ceph_mds_request *req,
 	 * will have trouble splicing in the virtual snapdir later
 	 */
 	if (rinfo->head->is_dentry && !req->r_aborted &&
+	    req->r_locked_dir &&
 	    (rinfo->head->is_target || strncmp(req->r_dentry->d_name.name,
 					       fsc->mount_options->snapdir_name,
 					       req->r_dentry->d_name.len))) {