librbd/object_map: potentially use in-memory object map in DiffRequest

librbd/object_map: potentially use in-memory object map in DiffRequest

If the object map for the end version is around (already loaded in
memory, either due to the end version being a snapshot or due to
exclusive lock being held), use it to run diff-iterate against the
beginning of time.  Since it's the only object map needed in that
case, such calls would be satisfied locally.

Fixes: https://tracker.ceph.com/issues/63341
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>