Fix Xen expt bridge forwarding with bridge-nf-call-iptables on newer dom0 kernels.

Something changed at Xen 4.16/Linux 5.15, where if the
net.bridge.bridge-nf-call-iptables sysctl is enabled (which we
do so that we can firewall/nat the control net bridge ifaces),
iptables rules apply to the expt net bridges as well.  This
seems to be new behavior, because although we set the default
global FORWARD chain policy to DROP, and do not create
iptables rules for expt net bridges, in Xen 4.11/Linux 5.4,
the bridge forwarded ip packets just fine.  In Xen 4.16/Linux
5.15, this behavior has changed, and ip traffic is not
forwarded unless we add this basic allow-all forwarding rule
for each expt bridge.