net: introduce NETDEV_CHANGE_TX_QUEUE_LEN

This patch introduces a new event - NETDEV_CHANGE_TX_QUEUE_LEN, this
will be triggered when tx_queue_len. It could be used by net device
who want to do some processing at that time. An example is tun who may
want to resize tx array when tx_queue_len is changed.

Cc: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index cfed7bc..a9e3805 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -1927,11 +1927,19 @@
 
 	if (tb[IFLA_TXQLEN]) {
 		unsigned long value = nla_get_u32(tb[IFLA_TXQLEN]);
+		unsigned long orig_len = dev->tx_queue_len;
 
-		if (dev->tx_queue_len ^ value)
+		if (dev->tx_queue_len ^ value) {
+			dev->tx_queue_len = value;
+			err = call_netdevice_notifiers(
+			      NETDEV_CHANGE_TX_QUEUE_LEN, dev);
+			err = notifier_to_errno(err);
+			if (err) {
+				dev->tx_queue_len = orig_len;
+				goto errout;
+			}
 			status |= DO_SETLINK_NOTIFY;
-
-		dev->tx_queue_len = value;
+		}
 	}
 
 	if (tb[IFLA_OPERSTATE])