net: Move config NET_NS to from net/Kconfig to init/Kconfig

Make NET_NS available underneath the generic Namespaces config option
since all of the other namespace options are there.

Signed-off-by: Matt Helsley <matthltc@us.ibm.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>