xenbus: Add proper handling of XS_ERROR from Xenbus for transactions.

If Xenstore sends back a XS_ERROR for TRANSACTION_END, the driver BUGs
because it cannot find the matching transaction in the list.  For
TRANSACTION_START, it leaks memory.

Check the message as returned from xenbus_dev_request_and_reply(), and
clean up for TRANSACTION_START or discard the error for
TRANSACTION_END.

Signed-off-by: Jennifer Herbert <Jennifer.Herbert@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

drivers/xen/xenbus/xenbus_dev_frontend.c

@@ -326,10 +326,13 @@ static int xenbus_write_transaction(unsigned msg_type,
 	}
 
 	if (msg_type == XS_TRANSACTION_START) {
-		trans->handle.id = simple_strtoul(reply, NULL, 0);
-
-		list_add(&trans->list, &u->transactions);
-	} else if (msg_type == XS_TRANSACTION_END) {
+		if (u->u.msg.type == XS_ERROR)
+			kfree(trans);
+		else {
+			trans->handle.id = simple_strtoul(reply, NULL, 0);
+			list_add(&trans->list, &u->transactions);
+		}
+	} else if (u->u.msg.type == XS_TRANSACTION_END) {
 		list_for_each_entry(trans, &u->transactions, list)
 			if (trans->handle.id == u->u.msg.tx_id)
 				break;