netns: Use net_eq() to compare net-namespaces for optimization.

Without CONFIG_NET_NS, namespace is always &init_net.
Compiler will be able to omit namespace comparisons with this patch.

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