qa/*/osd-backfill-recovery-log.sh: flush_pg_stats before checking log length

qa/*/osd-backfill-recovery-log.sh: flush_pg_stats before checking log length

It is possible for the pg dump to not be the latest when we check for newprimary
in _common_test(). This is because mgr_stats_period is 5 seconds, and we may not
have fetched the latest stats just yet. This causes the test to look at the same
stats before and after wait_for_clean.

Fixes: https://tracker.ceph.com/issues/43807 (2)
Signed-off-by: Neha Ojha <nojha@redhat.com>