librbd: don't stop at the first unremovable image when purging

librbd: don't stop at the first unremovable image when purging

As there is no inherent ordering, there may be multiple removable
images past the unremovable image.  On top of that, removing a clone
may make its parent removable so perform an additional pass if any
image gets removed.

Fixes: https://tracker.ceph.com/issues/51021
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 16d9a68a3e863b5a819860abf0696fb76fc9341a)

Conflicts:
	qa/workunits/rbd/cli_generic.sh [ commit 6e1434eefc3d
	  ("librbd: optionally move parent image to trash on remove")
	  not in nautilus ]