batman-adv: Fix lock assert after fragmentation change

The automatic recalculation of the maximum allowed MTU is usually triggered
by code sections which are already rtnl lock protected by callers outside
of batman-adv. But when the fragmentation setting is changed via
batman-adv's own batadv genl family, then the rtnl lock is not yet taken.

But dev_set_mtu requires that the caller holds the rtnl lock because it
uses netdevice notifiers. And this code will then fail the check for this
lock:

  RTNL: assertion failed at net/core/dev.c (1953)

Fixes: 12577be7 ("batman-adv: Merge bugfixes from 2023.2")
Signed-off-by: NSven Eckelmann <sven@narfation.org>

batman-adv/patches/0020-batman-adv-Fix-batadv_v_ogm_aggr_send-memory-leak.patch

+From: Sven Eckelmann <sven@narfation.org>
+Date: Mon, 21 Aug 2023 21:48:48 +0200
+Subject: batman-adv: Hold rtnl lock during MTU update via netlink
+
+The automatic recalculation of the maximum allowed MTU is usually triggered
+by code sections which are already rtnl lock protected by callers outside
+of batman-adv. But when the fragmentation setting is changed via
+batman-adv's own batadv genl family, then the rtnl lock is not yet taken.
+
+But dev_set_mtu requires that the caller holds the rtnl lock because it
+uses netdevice notifiers. And this code will then fail the check for this
+lock:
+
+  RTNL: assertion failed at net/core/dev.c (1953)
+
+Cc: stable@vger.kernel.org
+Reported-by: syzbot+f8812454d9b3ac00d282@syzkaller.appspotmail.com
+Fixes: 27c4d7c1c7fa ("batman-adv: Trigger events for auto adjusted MTU")
+Reviewed-by: Simon Horman <horms@kernel.org>
+Signed-off-by: Sven Eckelmann <sven@narfation.org>
+Origin: upstream, https://git.open-mesh.org/batman-adv.git/commit/aeb35331aa9a17f9affd84c1a5b020aeb4a976f4
+
+--- a/net/batman-adv/netlink.c
++++ b/net/batman-adv/netlink.c
+@@ -494,7 +494,10 @@ static int batadv_netlink_set_mesh(struc
+ 		attr = info->attrs[BATADV_ATTR_FRAGMENTATION_ENABLED];
+ 
+ 		atomic_set(&bat_priv->fragmentation, !!nla_get_u8(attr));
++
++		rtnl_lock();
+ 		batadv_update_min_mtu(bat_priv->soft_iface);
++		rtnl_unlock();
+ 	}
+ 
+ 	if (info->attrs[BATADV_ATTR_GW_BANDWIDTH_DOWN]) {