PG: only complete replicas should count toward min_size

Backport: emperor,dumpling,cuttlefish
Fixes: #7805
Signed-off-by: Samuel Just <sam.just@inktank.com>
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 0d5d3d1a30685e7c47173b974caa12076c43a9c4)