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>
(cherry picked from commit 0c4bb58c900efa2356ea8526d3432b2922787afa)

Conflicts:
	src/test/librbd/mock/MockObjectMap.h [ commit 87459c23aa05
	  ("librbd,rbd_mirror: do not include RWLock.h unless it is
	  used") not in pacific ]