Revert "ipv6: Don't depend on per socket memory for neighbour discovery messages"

Revert "ipv6: Don't depend on per socket memory for neighbour discovery messages"

This reverts commit a4257ad8c4b55290b6924b110909fd3314fa2e6f.

It seems to cause regressions, and in particular the output path
really depends upon there being a socket attached to skb->sk for
checks such as sk_mc_loop(skb->sk) for example.  See ip6_output_finish2().

Reported-by: Stephen Warren <swarren@wwwdotorg.org>
Reported-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>