alx: acquire mutex for alx_reinit in alx_change_mtu

alx_reinit has a lockdep assertion that the alx->mtx mutex must be held.
alx_reinit is called from two places: alx_reset and alx_change_mtu.
alx_reset does acquire alx->mtx before calling alx_reinit.
alx_change_mtu does not acquire this mutex, nor do its callers or any
path towards alx_change_mtu.
Acquire the mutex in alx_change_mtu.

The issue was introduced when the fine-grained locking was introduced
to the code to replace the RTNL. The same commit also introduced the
lockdep assertion.

Fixes: 4a5fe57 ("alx: use fine-grained locking instead of RTNL")
Signed-off-by: Niels Dossche <dossche.niels@gmail.com>
Link: https://lore.kernel.org/r/20220310232707.44251-1-dossche.niels@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/atheros/alx/main.c

@@ -1181,8 +1181,11 @@ static int alx_change_mtu(struct net_device *netdev, int mtu)
 	alx->hw.mtu = mtu;
 	alx->rxbuf_size = max(max_frame, ALX_DEF_RXBUF_SIZE);
 	netdev_update_features(netdev);
-	if (netif_running(netdev))
+	if (netif_running(netdev)) {
+		mutex_lock(&alx->mtx);
 		alx_reinit(alx);
+		mutex_unlock(&alx->mtx);
+	}
 	return 0;
 }