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