[NET]: Mark the setup_net as __net_init

The setup_net is called for the init net namespace
only (int the CONFIG_NET_NS=n of course) from the __init
function, so mark it as __net_init to disappear with the
caller after the boot.

Yet again, in the perfect world this has to be under
#ifdef CONFIG_NET_NS, but it isn't guaranteed that every
subsystem is registered *after* the init_net_ns is set
up. After we are sure, that we don't start registering
them before the init net setup, we'll be able to move
this code under the ifdef.

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