network: make all iptables functions used only in network_iptables.c static

network: make all iptables functions used only in network_iptables.c static

Now that the toplevel iptables functions have been moved out of the
linux bridge driver into network_iptables.c, all of the utility
functions are used only within that same file, so simplify it.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>