osd: automatically scrub PGs with invalid stats

If a PG has recnetly split and has invalid stats, scrub it now, even if
it has scrubbed recently.  This helps the stats become valid again soon.

Fixes: #8147
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1738 from ceph/wip-8147

osd: automatically scrub PGs with invalid stats

Reviewed-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>