osd/scrub: restart snap trimming only after scrubbing is done

osd/scrub: restart snap trimming only after scrubbing is done

Snap trimming that was postponed as the target PG was scrubbing
must be restarted at scrub completion.
PR #38111 moved trimming restart to just before the scrub fully
terminated. The current PR fixes that.

Trimming is also restarted in those cases where scrub was
queued but aborted immediately.

Fixes: https://tracker.ceph.com/issues/52026
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>