[IPV6]: NDISC: ensure responding to NS without link-layer information

[IPV6]: NDISC: ensure responding to NS without link-layer information

When sending NA in response to NS, we may not know the
link-layer address for the destination of the NA
since unicast NS is not required to include its link-layer information.
In this case, we first need to resolve the link-layer address.
(RFC 2461 7.2.4)

We now create neighbour entry for the destination
and link-layer information will be automatically solved
in the output path.

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