ndisc: Defer building IPv6 header.

Build ICMPv6 message first and make buffer management easier;
we can use skb->len when filling checksum in ICMPv6 header,
and then build IP header with length field.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>