PG::build_might_have_unfound: check pg_whomai, not osd whoami

Otherwise, we might skip (2,0) when we are (2,1).

Fixes: #7732
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1468 from ceph/wip-7732

PG::build_might_have_unfound: check pg_whomai, not osd whoami

Reviewed-by: Sage Weil <sage@inktank.com>