[IPV6] NDISC: Handle NDP messages to proxied addresses.

It is required to respond to NDP messages sent directly to the "target"
unicast address.  Proxying node (router) is required to handle such
messages.  To achieve this, check if the packet in forwarding patch is
NDP message.

With this patch, the proxy neighbor entries are always looked up in
forwarding path.  We may want to optimize further.

Based on MIPL2 kernel patch.

Signed-off-by: Ville Nuorvala <vnuorval@tcs.hut.fi>
Signed-off-by: Masahide NAKAMURA <nakam@linux-ipv6.org>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>