flow_dissector: Add number of vlan tags dissector

Our customers in the fiber telecom world have network configurations
where they would like to control their traffic according to the number
of tags appearing in the packet.

For example, TR247 GPON conformance test suite specification mostly
talks about untagged, single, double tagged packets and gives lax
guidelines on the vlan protocol vs. number of vlan tags.

This is different from the common IT networks where 802.1Q and 802.1ad
protocols are usually describe single and double tagged packet. GPON
configurations that we work with have arbitrary mix the above protocols
and number of vlan tags in the packet.

The goal is to make the following TC commands possible:

tc filter add dev eth1 ingress flower \
  num_of_vlans 1 vlan_prio 5 action drop

From our logs, we have redirect rules such that:

tc filter add dev $GPON ingress flower num_of_vlans $N \
     action mirred egress redirect dev $DEV

where N can range from 0 to 3 and $DEV is the function of $N.

Also there are rules setting skb mark based on the number of vlans:

tc filter add dev $GPON ingress flower num_of_vlans $N vlan_prio \
    $P action skbedit mark $M

This new dissector allows extracting the number of vlan tags existing in
the packet.

Signed-off-by: Boris Sukholitko <boris.sukholitko@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>