net_sched: add tcf_kfree_skb_list() helper

Using kfree_skb_list_reason() to free list of skbs from qdisc
operations seems wrong as each skb might have a different drop reason.

Cleanup __dev_xmit_skb() to call tcf_kfree_skb_list() once
in preparation of the following patch.

Signed-off-by: Eric Dumazet <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

include/net/sch_generic.h

···
 	tc_skb_cb(skb)->drop_reason = reason;
 }
 
+static inline void tcf_kfree_skb_list(struct sk_buff *skb)
+{
+	while (unlikely(skb)) {
+		struct sk_buff *next = skb->next;
+
+		prefetch(next);
+		kfree_skb_reason(skb, tcf_get_drop_reason(skb));
+		skb = next;
+	}
+}
+
 /* Instead of calling kfree_skb() while root qdisc lock is held,
  * queue the skb for future freeing at end of __dev_xmit_skb()
  */

net/core/dev.c

···
 				__qdisc_run(q);
 				qdisc_run_end(q);
 
-				goto no_lock_out;
+				goto free_skbs;
 			}
 
 			qdisc_bstats_cpu_update(q, skb);
···
 
 		rc = dev_qdisc_enqueue(skb, q, &to_free, txq);
 		qdisc_run(q);
-
-no_lock_out:
-		if (unlikely(to_free))
-			kfree_skb_list_reason(to_free,
-					      tcf_get_drop_reason(to_free));
-		return rc;
+		goto free_skbs;
 	}
 
 	/* Open code llist_add(&skb->ll_node, &q->defer_list) + queue limit.
···
 	}
 unlock:
 	spin_unlock(root_lock);
-	if (unlikely(to_free))
-		kfree_skb_list_reason(to_free,
-				      tcf_get_drop_reason(to_free));
+
+free_skbs:
+	tcf_kfree_skb_list(to_free);
 	return rc;
 }