allow vlan(4) interfaces to be created on a bridge(4)

[ivy]

Summary

A vlan interface on top of a bridge will act as a layer 3 port for
bridge traffic on that vlan, sometimes called an "SVI". This allows
the host to send/receive traffic on that vlan without having to create
a separate epair(4) and vlan(4) to tag and untag the traffic.

[kp]

sys/net/if_bridge.c

431

I think if_vlan_var.h is an appropriate spot for that. It already has vlan_trunkdev_p anyway.
if_ethersubr.c already includes it, so we're not going to be causing any include pollution.

2488

vlan = DOT1Q_VID_NULL ?

[ivy]

update for new vlan filtering behaviour

[ivy]

sys/net/if_bridge.c

431

done in D50567.

2488

this code was unnecessarily complicated, i've simplified it and removed the default assignment.

[zlei]

Generally looks good to me.

sys/net/if_bridge.c

850

This appears to be redundant. In if_vlan.c you have already tested the interface type.

	if (p->if_type != IFT_ETHER &&
	    p->if_type != IFT_L2VLAN &&
+++	    p->if_type != IFT_BRIDGE &&
	    (p->if_capenable & IFCAP_VLAN_HWTAGGING) == 0)

[ivy]

sys/net/if_bridge.c

850

i think we should still set the capability here in case some other interface wants to send us tagged frames. (epair perhaps? although i haven't checked this.)

in general, if_bridge does support "hardware" tagging, so the cap seems correct. perhaps the check could be removed from if_vlan though.