Disable netfilter on bridges on SUSE

For details, see
http://docs.openstack.org/admin-guide-cloud/content/ch_introduction-to-openstack-compute.html
("Packet loss from instances to nova-network server (VLANManager mode)")
and https://bugzilla.redhat.com/show_bug.cgi?id=512206

Upstream recommends touching sysctl settings, but that won't work if the
bridge module is not loaded yet, hence using the modprobe trick.