Since skb_checksum_help has been using pskb_expand_head for a while
now without any ill effects, I thought it would be a good idea to
remove the double pointers from it and its callers.

This is what the following patch does.  The only 'rider' bit is the
removal of an unnecessary BUG_ON in ip6_pkt_discard_out.  The preceding
assignment was only added because the following function oopsed so
there is no point in doing BUG_ON.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>