osd: fix osd_tp shutdown

We need to clear the queue, not just drain the currently executing jobs.

Fixes: #9218
Backport: firefly
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2335 from ceph/wip-9218

osd: fix osd_tp shutdown

Reviewed-by: Samuel Just <sam.just@inktank.com>