[NET]: Preliminaty annotation of skb->csum.

It's still not completely right; we need to split it into anon unions
of __wsum and unsigned - for cases when we use it for partial checksum
and for offset of checksum in skb

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -273,8 +273,8 @@ struct sk_buff {
 
 	unsigned int		len,
 				data_len,
-				mac_len,
-				csum;
+				mac_len;
+	__wsum			csum;
 	__u32			priority;
 	__u8			local_df:1,
 				cloned:1,