rbd: support for object-map and fast-diff

Speed up reads, discards and zeroouts through RBD_OBJ_FLAG_MAY_EXIST
and RBD_OBJ_FLAG_NOOP_FOR_NONEXISTENT based on object map.

Invalid object maps are not trusted, but still updated.  Note that we
never iterate, resize or invalidate object maps.  If object-map feature
is enabled but object map fails to load, we just fail the requester
(either "rbd map" or I/O, by way of post-acquire action).

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

include/linux/ceph/cls_lock_client.h

@@ -52,4 +52,7 @@ int ceph_cls_lock_info(struct ceph_osd_client *osdc,
 		       char *lock_name, u8 *type, char **tag,
 		       struct ceph_locker **lockers, u32 *num_lockers);
 
+int ceph_cls_assert_locked(struct ceph_osd_request *req, int which,
+			   char *lock_name, u8 type, char *cookie, char *tag);
+
 #endif