skb_copy_bits() can return err

diff -urN xen-unstable-0601/linux-2.6.11-xen-sparse/drivers/xen/netback/netback.c niv-unstable-0601/linux-2.6.11-xen-sparse/drivers/xen/netback/netback.c
@@ -153,7 +153,9 @@
             goto drop;
         skb_reserve(nskb, hlen);
         __skb_put(nskb, skb->len);
-        (void)skb_copy_bits(skb, -hlen, nskb->data - hlen, skb->len + hlen);
+        if (skb_copy_bits(skb, -hlen, nskb->data - hlen, skb->len + hlen)) {
+		BUG();
+	}
         nskb->dev = skb->dev;
         nskb->proto_csum_valid = skb->proto_csum_valid;
         dev_kfree_skb(skb);