PG: only complete replicas should count toward min_size

Backport: emperor,dumpling,cuttlefish
Fixes: #7805
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1517 from ceph/wip-7805

PG: only complete replicas should count toward min_size

Reviewed-by: Sage Weil <sage@inktank.com>