bridge: default to not filtering L3

Change the default for net.link.bridge.pfil_member and
net.link.bridge.pfil_bridge to zero.

That is, default to not calling layer 3 firewalls on the bridge or its
member interfaces.

With either of these enabled the bridge will, during L2 processing,
remove the Ethernet header from packets, feed them to L3 firewalls,
re-add the Ethernet header and send them out.

Not only does this interact very poorly with firewalls which defer
packets, or reassemble and refragment IPv6, it also causes considerable
confusion for users, because the firewall gets called in unexpected
ways.

For example, a bridge which contains a bhyve tap and the host's LAN
interface. We'd expect traffic between the LAN and bhyve VM to pass, no
matter what (layer 3) firewall rules are set on the host. That's not the
case as long as pfil_bridge or pfil_member are set.

Reviewed by: Zhenlei Huang
MFC: never
Differential Revision: https://reviews.freebsd.org/D37009