dsa: qca8k: Use nested lock to avoid splat

qca8k_phy_eth_command() is used to probe the child MDIO bus while the
parent MDIO is locked. This causes lockdep splat, reporting a possible
deadlock. It is not an actually deadlock, because different locks are
used. By making use of mutex_lock_nested() we can avoid this false
positive.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20241110175955.3053664-1-andrew@lunn.ch
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/qca/qca8k-8xxx.c

@@ -673,7 +673,7 @@ qca8k_phy_eth_command(struct qca8k_priv *priv, bool read, int phy,
 	 * We therefore need to lock the MDIO bus onto which the switch is
 	 * connected.
 	 */
-	mutex_lock(&priv->bus->mdio_lock);
+	mutex_lock_nested(&priv->bus->mdio_lock, MDIO_MUTEX_NESTED);
 
 	/* Actually start the request:
 	 * 1. Send mdio master packet