librbd/mirror: tweak which snapshot is unlinked when at capacity

librbd/mirror: tweak which snapshot is unlinked when at capacity

The rbd-mirror daemon will attempt to sync from the last synced
snapshot to the next mirror snapshot. When the limit is at 3, this
currently can result in a situation where an in-use sync snapshot is
deleted. Instead of unlinking the second oldest snapshot, always
unlink the third oldest.

Fixes: https://tracker.ceph.com/issues/48553
Signed-off-by: Jason Dillaman <dillaman@redhat.com>