ceph: fix MDS random selection readiness predicate

ceph: fix MDS random selection readiness predicate

CEPH_MDS_IS_READY() is parsed so that the ternary expression can
return true for an MDS entry with state 0 when it is not laggy. This
allows the random selector to choose a down/DNE rank.

Group the ternary expression under the state check so zero-state ranks
are not treated as ready.

Cc: stable@vger.kernel.org
Fixes: b38c9eb4757d ("ceph: add possible_max_rank and make the code more readable")
Link: https://tracker.ceph.com/issues/78648
Signed-off-by: Yiming Zhu <zhuyiming@kuaishou.com>
Reviewed-by: Viacheslav Dubeyko <slava@dubeyko.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>