ceph: add missing locking to protect i_snap_realm_item during split

All ci->i_snap_realm_item/realm->inodes_with_caps manipulation should be
protected by realm->inodes_with_caps_lock.  This bug would have only bit
us in a rare race with a realm split (during some snap creations).
Signed-off-by: Sage Weil <sage@newdream.net>

fs/ceph/snap.c

@@ -818,7 +818,9 @@ void ceph_handle_snap(struct ceph_mds_client *mdsc,
 			 * queued (again) by ceph_update_snap_trace()
 			 * below.  Queue it _now_, under the old context.
 			 */
+			spin_lock(&realm->inodes_with_caps_lock);
 			list_del_init(&ci->i_snap_realm_item);
+			spin_unlock(&realm->inodes_with_caps_lock);
 			spin_unlock(&inode->i_lock);
 
 			ceph_queue_cap_snap(ci,