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>
(cherry picked from commit 54f47cc28ffd2d29b4f8cfaf56a5a5be2909bde7)

Conflicts:
	src/include/rbd/librbd.h [ commit e5ccce14c4b0 ("rbd: add group
	  snap info command") not in squid ]