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>
(cherry picked from commit 68b440d66539e820c9ce86a6942c3188be4ee1ec)