librbd: add rbd_diff_iterate3() API to take source snapshot by ID

librbd: add rbd_diff_iterate3() API to take source snapshot by ID

Allow a diff to start from a non-user snapshot.  This would be used by
"rbd du" command to account for non-user snapshots which are currently
just skipped potentially resulting in underreported space usage and in
other places.

Fixes: https://tracker.ceph.com/issues/65720
Co-authored-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Vinay Bhaskar Varada <vvarada@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>