[NETNS]: Don't memset() netns to zero manually

The newly created net namespace is set to 0 with memset()
in setup_net(). The setup_net() is also called for the
init_net_ns(), which is zeroed naturally as a global var.

So remove this memset and allocate new nets with the
kmem_cache_zalloc().

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>