mon/OSDMonitor: report pg[pgp]_num_target instead of pg[pgp]_num

mon/OSDMonitor: report pg[pgp]_num_target instead of pg[pgp]_num

As by calling "ceph osd pool set <pool-name> pg_num", we set the
pg_num_target instead. Hence we should report pg_num_target as well
when invoked by the "ceph osd pool get <pool-name> pg_num" call.

I doubt an ideal fix would be exposing both the pg_num(*actual*) and
pg_num_target concepts to user, but for now I think we can stop here
and get some more feedback first.

Fixes: http://tracker.ceph.com/issues/40193
Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>