net: ipv6: Add packet queueing while waiting NA

If we do not know the neighbor link address, we need to send
neighbor solicitation message to net. While waiting neighbor
advertisement, there might be other packets that we want to
send to that neighbor. Queue those packets so that all of them
can be sent when NA message is received.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>