SLM: max/after and max/failed fix (#68352)

Combining both maximum and expire after retention configuration did not
compose well in that, as long as there are any snapshots to delete due
to the maximum criteria, the expire after criteria was not evaluated,
fixed to consider both.

When evaluating the maximum criteria, a number of snapshots to delete
was calculated based on successful snapshots. However, when evaluating
which snapshot is eligible for deletion, the entire list was used,
resulting in deleting too few snapshots.