xenbus: don't look up transaction IDs for ordinary writes

This should really only be done for XS_TRANSACTION_END messages, or
else at least some of the xenstore-* tools don't work anymore.

Fixes: 0beef634 ("xenbus: don't BUG() on user mode induced condition")
Reported-by: NRichard Schütz <rschuetz@uni-koblenz.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: NJan Beulich <jbeulich@suse.com>
Tested-by: NRichard Schütz <rschuetz@uni-koblenz.de>
Signed-off-by: NDavid Vrabel <david.vrabel@citrix.com>

drivers/xen/xenbus/xenbus_dev_frontend.c

@@ -316,7 +316,7 @@ static int xenbus_write_transaction(unsigned msg_type,
 			rc = -ENOMEM;
 			goto out;
 		}
-	} else {
+	} else if (msg_type == XS_TRANSACTION_END) {
 		list_for_each_entry(trans, &u->transactions, list)
 			if (trans->handle.id == u->u.msg.tx_id)
 				break;