libceph: Fix sparse warning

Make ceph_monc_do_poolop() static to remove the following sparse warning:
 * net/ceph/mon_client.c:616:5: warning: symbol 'ceph_monc_do_poolop' was not
   declared. Should it be static?
Also drops the 'ceph_monc_' prefix, now being a private function.

Signed-off-by: Iulius Curt <icurt@ixiacom.com>
Signed-off-by: Sage Weil <sage@inktank.com>