netfront: use skb.cb for storing private data

netfront: use skb.cb for storing private data

Netfront's use of nh.raw and h.raw for storing page+offset is a bit
hinky, and it breaks with upcoming network stack updates which reduce
these fields to sub-pointer sizes.  Fortunately, skb offers the "cb"
field specifically for stashing this kind of info, so use it.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Chris Wright <chrisw@sous-sol.org>
Cc: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>