osd/PG: fix crash from second backfill reservation rejection

If we get more than one reservation rejection we should ignore them; when
we got the first we already sent out cancellations.  More importantly, we
should not crash.

Fixes: #8863
Backport: firefly, dumpling
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 2b13de16c522754e30a0a55fb9d072082dac455e)