ceph: add possible_max_rank and make the code more readable

The m_num_mds here is actually the number for MDSs which are in
up:active status, and it will be duplicated to m_num_active_mds,
so remove it.

Add possible_max_rank to the mdsmap struct and this will be
the correctly possible largest rank boundary.

Remove the special case for one mds in __mdsmap_get_random_mds(),
because the validate mds rank may not always be 0.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>