net: dsa: tag_8021q: only call skb_push/skb_pull around __skb_vlan_pop

__skb_vlan_pop() needs skb->data to point at the mac_header, while
skb_vlan_tag_present() and skb_vlan_tag_get() don't, because they don't
look at skb->data at all.

So we can avoid uselessly moving around skb->data for the case where the
VLAN tag was offloaded by the DSA master.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20220215204722.2134816-1-vladimir.oltean@nxp.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/dsa/tag_8021q.c

@@ -577,14 +577,14 @@ void dsa_8021q_rcv(struct sk_buff *skb, int *source_port, int *switch_id)
 {
 	u16 vid, tci;
 
-	skb_push_rcsum(skb, ETH_HLEN);
 	if (skb_vlan_tag_present(skb)) {
 		tci = skb_vlan_tag_get(skb);
 		__vlan_hwaccel_clear_tag(skb);
 	} else {
+		skb_push_rcsum(skb, ETH_HLEN);
 		__skb_vlan_pop(skb, &tci);
-	}
 		skb_pull_rcsum(skb, ETH_HLEN);
+	}
 
 	vid = tci & VLAN_VID_MASK;