Update 'confirmed' timestamp when ARP packet is received. It shouldn't
affect locktime logic and anyway entry can be confirmed by any higher-layer
protocol. Thus it makes to sense not to confirm it when ARP packet is
received.[...]
Fixes: 77d7123342 ("neighbour: update neigh timestamps iff update is
effective")
Signed-off-by: Vasily Khoruzhick <vasilykh@xxxxxxxxxx>