osd: allow io priority to be set for the disk_tp

The disk_tp covers scrubbing, pg deletion, and snap trimming

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 84b3003119eeb8acfb3faacf357e6c6a452950e3)

Conflicts:
	src/osd/OSD.cc

(cherry picked from commit 987ad133415aa988061c95259f9412b05ce8ac7e)
osd/ReplicatedPG: only block on backfill_pos if we got our backfill reservation

If we haven't reserved backfill yet, we may block indefinitely, in which case
we don't want to block the op at all.

Note that this was fixed in emperor and later when the code was refactored.
This is a minimal fix just for dumpling.

Fixes: #8595
Signed-off-by: Sage Weil <sage@inktank.com>