net: stmmac: Use rtnl_lock/unlock on netif_set_real_num_rx_queues() call

Fix an issue where dump stack is printed on suspend resume flow due to
netif_set_real_num_rx_queues() is not called with rtnl_lock held().

Fixes: 686cff3 ("net: stmmac: Fix incorrect location to set real_num_rx|tx_queues")
Reported-by: Christophe ROULLIER <christophe.roullier@st.com>
Tested-by: Christophe ROULLIER <christophe.roullier@st.com>
Cc: Alexandre TORGUE <alexandre.torgue@st.com>
Reviewed-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: Wong Vee Khee <vee.khee.wong@intel.com>
Link: https://lore.kernel.org/r/20201115074210.23605-1-vee.khee.wong@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: wvk86

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -5247,6 +5247,7 @@ int stmmac_resume(struct device *dev)
 			return ret;
 	}
 
+	rtnl_lock();
 	mutex_lock(&priv->lock);
 
 	stmmac_reset_queues_param(priv);
@@ -5262,6 +5263,7 @@ int stmmac_resume(struct device *dev)
 	stmmac_enable_all_queues(priv);
 
 	mutex_unlock(&priv->lock);
+	rtnl_unlock();
 
 	if (!device_may_wakeup(priv->device) || !priv->plat->pmt) {
 		rtnl_lock();