osd/scrub: make sched-targets comparator transitive

osd/scrub: make sched-targets comparator transitive

The comparator used to sort the scrub targets wasn't transitive, which
can lead to undefined behavior (and likely - to a crash of the sort
algorithm in some implementations). That - due to the clause comparing
the two targets of the same PG.

This commit fixes the comparator. The sorted queue is no longer
guaranteed to have a ripe deep target before the corresponding
shallow one, which is less than optimal - but it won't crash.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>