qa/standalone/osd/osd-backfill-stats: fixes

qa/standalone/osd/osd-backfill-stats: fixes

Grep from the primary's log, not every osd's log.

For the backfill_remapped task in particular, after the pg_temp change it
just so happens that the primary changes across the pool size change and
thus two different primaries do (some) backfill.  Fix that test to pass
the correct primary.

Other tests are unaffected as they do not (happen to) trigger a primary
change and already satisfied the (removed) check that only one OSD does
backfill.

Signed-off-by: Sage Weil <sage@redhat.com>