librbd/object_map: fix diff from snapshot when image is grown

librbd/object_map: fix diff from snapshot when image is grown

Commit 399a45e11332 ("librbd/object_map: rbd diff between two
snapshots lists entire image content") fixed most of the damage caused
by commit b81cd2460de7 ("librbd/object_map: diff state machine should
track object existence"), but the case of a "resize diff" when diffing
from snapshot was missed.  An area that was freshly allocated in image
resize is the same in principle as a freshly created image and objects
marked OBJECT_EXISTS_CLEAN are no exception.  Diff for such objects in
such an area should be set to DIFF_STATE_DATA_UPDATED, however
currently when diffing from snapshot, it's set to DIFF_STATE_DATA.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 34386d29a8b96de79c38465062b8e93e7fc6e184)