net: ipv6: Silently drop unwanted NA messages

Silently drop the IPv6 Neighbor Advertisement if we receive it
for an unknown neighbor or if there some some issue in the packet.
Returning error here would cause the ICMP module to print an
actual error which just pollutes the log without any apparent
benefit.

Fixes #66063

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>