udp: add udp gso

Implement generic segmentation offload support for udp datagrams. A
follow-up patch adds support to the protocol stack to generate such
packets.

UDP GSO is not UFO. UFO fragments a single large datagram. GSO splits
a large payload into a number of discrete UDP datagrams.

The implementation adds a GSO type SKB_UDP_GSO_L4 to differentiate it
from UFO (SKB_UDP_GSO).

IPPROTO_UDPLITE is excluded, as that protocol has no gso handler
registered.

[ Export __udp_gso_segment for ipv6. -DaveM ]

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index d274059..a4a5c0c 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -573,6 +573,8 @@ enum {
 	SKB_GSO_ESP = 1 << 15,
 
 	SKB_GSO_UDP = 1 << 16,
+
+	SKB_GSO_UDP_L4 = 1 << 17,
 };
 
 #if BITS_PER_LONG > 32