RDS: have sockets get transport module references

Right now there's nothing to stop the various paths that use
rs->rs_transport from racing with rmmod and executing freed transport
code.  The simple fix is to have binding to a transport also hold a
reference to the transport's module, removing this class of races.

We already had an unused t_owner field which was set for the modular
transports and which wasn't set for the built-in loop transport.

Signed-off-by: Zach Brown <zach.brown@oracle.com>

net/rds/af_rds.c

@@ -90,6 +90,8 @@ static int rds_release(struct socket *sock)
 	rds_sock_count--;
 	spin_unlock_irqrestore(&rds_sock_lock, flags);
 
+	rds_trans_put(rs->rs_transport);
+
 	sock->sk = NULL;
 	sock_put(sk);
 out: