[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>