net: Add priority to packet_offload objects.

When we scan a packet for GRO processing, we want to see the most
common packet types in the front of the offload_base list.

So add a priority field so we can handle this properly.

IPv4/IPv6 get the highest priority with the implicit zero priority
field.

Next comes ethernet with a priority of 10, and then we have the MPLS
types with a priority of 15.

Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Suggested-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1997,6 +1997,7 @@ struct offload_callbacks {
 
 struct packet_offload {
 	__be16			 type;	/* This is really htons(ether_type). */
+	u16			 priority;
 	struct offload_callbacks callbacks;
 	struct list_head	 list;
 };