[IPV6] NDISC: Avoid updating neighbor cache for proxied address in receiving NA.

This aims at proxying router not updating neighbor cache entry for proxied
address when it receives NA because either the proxied node is off link or
it has already sent a NA to the proxied router.

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>