1. We add default PCP because some of our apps may not want to set PCP
manually (e.g. Our user may just ping with pre-set PCP)
2. The `vlan_qos` is used as PCP when setting Linux's priority mapping:
https://github.com/torvalds/linux/blob/v6.12/net/8021q/vlan.c#L590
Although `vlan_qos` is not used when creating VLAN on Linux, we can
use it as PCP on creating VLAN (without changing its meaning), and
keep compatible with Linux's creating (Exactly the same when
`vlan_qos` is 0).
Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>
Inspired by Linux's way, we also create VLAN devices for managing VLAN,
which will become interfaces like `eth0.58`.
QinQ is also supported, we can create VLAN devices above another VLAN
devices, like `eth0.100.101` (or even `eth0.1.2.3.4`, also supported on
Linux).
Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>
