osd/scrub: fix job requeue conditions

osd/scrub: fix job requeue conditions

Previous commits handled the following two cases correctly:
- requeueing a scrub job while the OSD is still the primary, and
- not restoring the scrub job to the queue if the PG is not there;
Here we handle the missed scenario: the PG is there (we were able to
lock it), but is no longer the primary.

Also - a configuration change must not cause a re-queue of a
scrub-job for a PG that is in the middle of scrubbing.

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