From: Sven Eckelmann <sven@narfation.org>
Date: Wed, 19 Jul 2023 10:15:05 +0200
Subject: batman-adv: Trigger events for auto adjusted MTU

If an interface changes the MTU, it is expected that an NETDEV_PRECHANGEMTU
and NETDEV_CHANGEMTU notification events is triggered. This worked fine for
.ndo_change_mtu based changes because core networking code took care of it.
But for auto-adjustments after hard-interfaces changes, these events were
simply missing.

Due to this problem, non-batman-adv components weren't aware of MTU changes
and thus couldn't perform their own tasks correctly.

Fixes: 8009e9f7ac4f ("set bat0 MTU according to the MTUs of the hard interfaces")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Origin: upstream, https://git.open-mesh.org/batman-adv.git/commit/27c4d7c1c7fa39d71ea6ccf1c23bcb4773243800

--- a/net/batman-adv/hard-interface.c
+++ b/net/batman-adv/hard-interface.c
@@ -629,7 +629,7 @@ out:
  */
 void batadv_update_min_mtu(struct net_device *soft_iface)
 {
-	soft_iface->mtu = batadv_hardif_min_mtu(soft_iface);
+	dev_set_mtu(soft_iface, batadv_hardif_min_mtu(soft_iface));
 
 	/* Check if the local translate table should be cleaned up to match a
 	 * new (and smaller) MTU.