librbd/object_map: resurrect diff-iterate behavior when image is shrunk

librbd/object_map: resurrect diff-iterate behavior when image is shrunk

The new "track the largest of all versions in the set, diff state is
only ever grown" semantics introduced in commit 330f2a7bb94f ("librbd:
helper state machine for computing diffs between object-maps") don't
make sense for diff-iterate.  It's a waste because DiffIterate won't
query beyond the end version size -- this is baked into the API.

Limit this behavior to deep-copy and resurrect the original behavior
from 2015 for diff-iterate.

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