flow_dissector: Do not count vlan tags inside tunnel payload

We've met the problem that when there is a vlan tag inside
GRE encapsulation, the match of num_of_vlans fails.
It is caused by the vlan tag inside GRE payload has been
counted into num_of_vlans, which is not expected.

One example packet is like this:
Ethernet II, Src: Broadcom_68:56:07 (00:10:18:68:56:07)
                   Dst: Broadcom_68:56:08 (00:10:18:68:56:08)
802.1Q Virtual LAN, PRI: 0, DEI: 0, ID: 100
Internet Protocol Version 4, Src: 192.168.1.4, Dst: 192.168.1.200
Generic Routing Encapsulation (Transparent Ethernet bridging)
Ethernet II, Src: Broadcom_68:58:07 (00:10:18:68:58:07)
                   Dst: Broadcom_68:58:08 (00:10:18:68:58:08)
802.1Q Virtual LAN, PRI: 0, DEI: 0, ID: 200
...
It should match the (num_of_vlans 1) rule, but it matches
the (num_of_vlans 2) rule.

The vlan tags inside the GRE or other tunnel encapsulated payload
should not be taken into num_of_vlans.
The fix is to stop counting the vlan number when the encapsulation
bit is set.

Fixes: 34951fcf ("flow_dissector: Add number of vlan tags dissector")
Signed-off-by: Qingqing Yang <qingqing.yang@broadcom.com>
Reviewed-by: Boris Sukholitko <boris.sukholitko@broadcom.com>
Link: https://lore.kernel.org/r/20220919074808.136640-1-qingqing.yang@broadcom.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/flow_dissector.c

@@ -1201,8 +1201,8 @@ bool __skb_flow_dissect(const struct net *net,
 			nhoff += sizeof(*vlan);
 		}
 
-		if (dissector_uses_key(flow_dissector,
-				       FLOW_DISSECTOR_KEY_NUM_OF_VLANS)) {
+		if (dissector_uses_key(flow_dissector, FLOW_DISSECTOR_KEY_NUM_OF_VLANS) &&
+		    !(key_control->flags & FLOW_DIS_ENCAPSULATION)) {
 			struct flow_dissector_key_num_of_vlans *key_nvs;
 
 			key_nvs = skb_flow_dissector_target(flow_dissector,