osd/PG: fix _finish_recovery vs repair race

osd/PG: fix _finish_recovery vs repair race

On detecting a corrupted object, primary may automatically
repair that object by leveraging the existing recovery procedure,
which turned out to be racy with a previous unfinished _finish_recovery
callback - the problem would then be that _finish_recovery might
continue to purge some strays that we still want to pull data from.

Fix by re-checking if there are any newly added missing objects when
executing _finish_recovery.

Note that before https://github.com/ceph/ceph/pull/29756 we might
instead have to call needs_recovery to catch the race condition
since we did not evict pg from clean state when triggering an auto-repair..

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>