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>

libceph, ceph: move ceph_calc_file_object_mapping() to striper.c

ceph_calc_file_object_mapping() has nothing to do with osdmaps.

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

libceph: striping framework implementation

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