Commit 868aabb4708d introduced per-flow priority. Modern network interfaces
can do hardware VLAN tag insertion. For untagged traffic, when the
interface is not enabled with PCP tagging, aka `ifconfig em0 -pcp`, it
happens to work due to driver bugs mentioned in D39499. From code review,
I can conclude that if the interface does not support hardware VLAN insertion
then it will not work.
Fix by checking flag M_VLANTAG which is set in mbuf packet header, so that
per-flow PCP for untagged traffic will work on both
1. Strict drivers that check hardware VLAN insertion
2. Network interfaces that do not support hardware VLAN insertion
Fixes: 868aabb4708d Add IP(V6)_VLAN_PCP to set 802.1 priority per-flow
MFC after: 1 week