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/transport.c

@@ -71,19 +71,28 @@ void rds_trans_unregister(struct rds_transport *trans)
 }
 EXPORT_SYMBOL_GPL(rds_trans_unregister);
 
+void rds_trans_put(struct rds_transport *trans)
+{
+	if (trans && trans->t_owner)
+		module_put(trans->t_owner);
+}
+
 struct rds_transport *rds_trans_get_preferred(__be32 addr)
 {
 	struct rds_transport *ret = NULL;
-	int i;
+	struct rds_transport *trans;
+	unsigned int i;
 
 	if (IN_LOOPBACK(ntohl(addr)))
 		return &rds_loop_transport;
 
 	down_read(&rds_trans_sem);
-	for (i = 0; i < RDS_TRANS_COUNT; i++)
-	{
-		if (transports[i] && (transports[i]->laddr_check(addr) == 0)) {
-			ret = transports[i];
+	for (i = 0; i < RDS_TRANS_COUNT; i++) {
+		trans = transports[i];
+
+		if (trans && (trans->laddr_check(addr) == 0) &&
+		    (!trans->t_owner || try_module_get(trans->t_owner))) {
+			ret = trans;
 			break;
 		}
 	}