macvtap: fix tx_dropped counting error

After commit 8ffab51b3dfc54876f145f15b351c41f3f703195
(macvlan: lockless tx path), tx stat counter were converted to percpu stat
structure. So we need use to this also for tx_dropped in macvtap. Otherwise, the
management won't notice the dropping packet in macvtap tx path.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Vlad Yasevich <vyasevic@redhat.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/macvtap.c

@@ -744,7 +744,7 @@ err:
 	rcu_read_lock();
 	vlan = rcu_dereference(q->vlan);
 	if (vlan)
-		vlan->dev->stats.tx_dropped++;
+		this_cpu_inc(vlan->pcpu_stats->tx_dropped);
 	rcu_read_unlock();
 
 	return err;