[SK_BUFF] ipconfig: Another conversion to skb_reset_network_header related to skb_put

boot_pkt->iph is the first member, that is at skb->data, so just use
skb_reset_network_header().

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