osd/PG: do not queue scrub if PG is not active when unblock

osd/PG: do not queue scrub if PG is not active when unblock

- we are scrubbing
- we block scrub on an object
- we have an interval reset on epoch X
- we unblock and queue scrub in epoch X
- we finish the reset (we are no longer scrubbing)
- we process the PGScrub on epoch X, it is still epoch X, and we are not
  scrubbing
  -> assert

Fix by not queueing scrub if we are unblocking and the PG is no longer
active (because we are mid-way through the peering reset).

Fixes: http://tracker.ceph.com/issues/40451
Signed-off-by: Sage Weil <sage@redhat.com>