xen-netback: don't "handle" error by BUG()

In particular -ENOMEM may come back here, from set_foreign_p2m_mapping().
Don't make problems worse, the more that handling elsewhere (together
with map's status fields now indicating whether a mapping wasn't even
attempted, and hence has to be considered failed) doesn't require this
odd way of dealing with errors.

This is part of XSA-362.
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: stable@vger.kernel.org
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/net/xen-netback/netback.c

@@ -1342,13 +1342,11 @@ int xenvif_tx_action(struct xenvif_queue *queue, int budget)
 		return 0;
 
 	gnttab_batch_copy(queue->tx_copy_ops, nr_cops);
-	if (nr_mops != 0) {
+	if (nr_mops != 0)
 		ret = gnttab_map_refs(queue->tx_map_ops,
 				      NULL,
 				      queue->pages_to_map,
 				      nr_mops);
-		BUG_ON(ret);
-	}
 
 	work_done = xenvif_tx_submit(queue);