Unbreak VLANs after r337943.

[np]

Summary

ether_set_pcp should not be called from ether_output_frame for VLAN
interfaces -- the vid + pcp will be inserted during vlan_transmit in
that case. r337943 sets the VLAN's ifnet's if_pcp to a proper PCP value
and this led to double encapsulation (once with vid 0 and second time
with vid+pcp).

[melifaro]

sys/net/if_ethersubr.c

464 ↗

(On Diff #47258)

Why do we check this in the dataplane part, instead of control plane?

[np]

sys/net/if_ethersubr.c

464 ↗

(On Diff #47258)

The tag needs to be inserted in the data path, and needs to work for both vid+pcp (ie if_vlan)
and only-pcp case ("real" ifnet). What kind of control plane check did you have in mind? I don't
see a way to avoid data plane changes.

I think kib@ wrote this (or at least worked on it last) and he may have additional insight.