ip: use ip_hdr() in __ip_make_skb() to retrieve IP header

skb->data already points to IP header, but for the sake of
consistency we can also use ip_hdr() to retrieve it.

Signed-off-by: Ansis Atteka <aatteka@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>