DNM: 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>
I vaguely remember these priorities living in variables larger than a __u8.. was it that the *Reserver classes use unsigned but the priority in teh backfill messages is a u8, or is it larger there?  If it's larger, maybe we should move to a large range.  I'm worried that if there are lots of PGs to recover we will run out of precision