ndisc: Do not try to update "updated" time if neighbour has already gone.

Commit 2152caea ("ipv6: Do not depend on rt->n in rt6_probe().")
introduce a bug to try to update "updated" time in neighbour
structure.
Update the "updated" time only if neighbour is available.

Bug was found by Dan Carpenter <dan.carpenter@oracle.com>

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>