osd: bump the priority of PG from OSD with most work

Based on the number of pending backfill reservations and current
backfills in the primary OSD, increase the priority of the backfill
reservation requests. This will increase the chances of reservations
from the busiest OSDs to succeed and probably shorten the tail of the
recovery process.

http://tracker.ceph.com/issues/9566 Fixes: #9566

Signed-off-by: Loic Dachary <loic-201408@dachary.org>