packets: fix compose_nd to not crash system

packets: fix compose_nd to not crash system

packet_set_ipv6 and packet_set_nd assumed that valid packets were already in
place, so they were not a good fit for composing new packets. In fact,
packet_set_ipv6 didn't even set the IP version or set L4 offset, causing crashes
when compose_nd tried to access L4.

This patch introduces ipv6_compose and uses it for compose_nd, then set the ND
packets without other helpers.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@redhat.com>