[SK_BUFF] xfrm4: use skb_reset_network_header

Setting it to skb->h.raw, which is valid, in the (to become) old pointer based
world order and in the new world of offset based layer headers.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>