librbd/object_map: add support for ranged diff-iterate

librbd/object_map: add support for ranged diff-iterate

Currently diff-iterate in fast-diff mode is performed on the entire
image no matter what image extent is passed to the API.  Then, unused
diff just gets discarded as DiffIterate ends up querying only objects
that the passed image extent maps to.  This hasn't been an issue for
internal consumers ("rbd du", "rbd diff", etc) because they work on the
entire image, but turns out to lead to quadratic slowdown in some QEMU
use cases.

0..UINT64_MAX range is carved out for deep-copy which is unranged by
definition.  To get effectively unranged diff-iterate, 0..UINT64_MAX-1
range can be used.

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