[PATCH] Log error in csum dev_queue_xmit error path

[Jon Mason <jdmason@us.ibm.com>]

While working bug #143, it came to my attention that no verbose error is
loged if the dom0 kernel mangles a packet and tries to checksum
offload it.  

This should let the user know what is going wrong (instead of silently
dropping the packet). 

Signed-off-by: Jon Mason <jdmason@us.ibm.com>

# HG changeset patch
# User root@opteron.ltc.austin.ibm.com
# Node ID 6e8be095dd8406da697b3956ba65ed724e55e50f
# Parent  f5b119533cc825726c9ecaa3bd2650b38033d4d8
Log an error if dom0 dev_queue_xmit receives a packet to be checksummed
that is not TCP/UDP.  Since we can only offload TCP/UDP checksums,
this error path should be entered.  If this is the case, domU checksum
offload needs to be diabled.

diff -r f5b119533cc8 -r 6e8be095dd84 linux-2.6-xen-sparse/net/core/dev.c
--- a/linux-2.6-xen-sparse/net/core/dev.c	Thu Dec  1 14:22:22 2005
+++ b/linux-2.6-xen-sparse/net/core/dev.c	Thu Dec  1 19:29:00 2005
@@ -1283,6 +1283,11 @@
 			skb->csum = offsetof(struct udphdr, check);
 			break;
 		default:
+			if (net_ratelimit())
+				printk(KERN_ERR "Attempting to checksum a "
+				       "non-TCP/UDP packet, dropping a protocol"
+				       " %d packet", skb->nh.iph->protocol);
+			rc = -EPROTO;
 			goto out_kfree_skb;
 		}
 		if ((skb->h.raw + skb->csum + 2) > skb->tail)