libceph: fix misjudgement of maximum monitor number

num_mon should allow up to CEPH_MAX_MON in ceph_monmap_decode().

Signed-off-by: Chengguang Xu <cgxu519@icloud.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>