[NETNS]: Register neighbour table parameters in the correct namespace.

neigh_sysctl_register should register sysctl entries inside correct namespace
to avoid naming conflict. Typical example is a loopback. Entries for it
present in all namespaces.

Required to make inetdev_event working.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>