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>