net/sched: em_meta: Fix 'meta vlan' to correctly recognize zero VID frames

commit d65f2fa6 upstream.

META_COLLECTOR int_vlan_tag() assumes that if the accel tag (vlan_tci)
is zero, then no vlan accel tag is present.

This is incorrect for zero VID vlan accel packets, making the following
match fail:
  tc filter add ... basic match 'meta(vlan mask 0xfff eq 0)' ...

Apparently 'int_vlan_tag' was implemented prior VLAN_TAG_PRESENT was
introduced in 05423b24 "vlan: allow null VLAN ID to be used"
(and at time introduced, the 'vlan_tx_tag_get' call in em_meta was not
 adapted).

Fix, testing skb_vlan_tag_present instead of testing skb_vlan_tag_get's
value.

Fixes: 05423b24 ("vlan: allow null VLAN ID to be used")
Fixes: 1a31f204 ("netsched: Allow meta match on vlan tag on receive")
Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: s/skb_vlan_tag/vlan_tx_tag/]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/sched/em_meta.c

@@ -176,11 +176,12 @@ META_COLLECTOR(int_vlan_tag)
 {
 	unsigned short tag;
 
-	tag = vlan_tx_tag_get(skb);
-	if (!tag && __vlan_get_tag(skb, &tag))
-		*err = -1;
-	else
+	if (vlan_tx_tag_present(skb))
+		dst->value = vlan_tx_tag_get(skb);
+	else if (!__vlan_get_tag(skb, &tag))
 		dst->value = tag;
+	else
+		*err = -1;
 }