[IPV6]: Reset the network header in ip6_nd_hdr

ip6_nd_hdr is always called immediately after a alloc_skb + skb_reserve
sequence, i.e. when skb->tail is equal to skb->data, making it correct to use
skb_reset_network_header().

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