PG::choose_acting: in mixed cluster case, acting may include backfill

Fixes: 9696
Backport: firefly, giant
Introduced: 92cfd370395385ca5537b5bc72220934c9f09026
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #2684 from ceph/wip-9696

PG::choose_acting: in mixed cluster case, acting may include backfill

Reviewed-by: Sage Weil <sage@redhat.com>