PG: skip pg_whoami.osd, not pg_whoami.shard in scrub feature check

Caused by typo in 68184d4574cd507ab3a065693e392917b98e16ec.

Fixes: #7517
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1307 from ceph/wip-7517

Wip 7517