i40evf: check rings before freeing resources

BugLink: http://bugs.launchpad.net/bugs/1536474

If the driver gets unloaded during reset recovery, it's possible
that it will attempt to free resources when they're already free.

Add a check to make sure that the Tx and Rx rings actually exist
before dereferencing them to free resources.

Change-ID: I4d2b7e9ede49f634d421a4c5deaa5446bc755eee
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
(cherry picked from commit fdb47ae8)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/net/ethernet/intel/i40evf/i40evf_main.c

@@ -2034,6 +2034,9 @@ void i40evf_free_all_tx_resources(struct i40evf_adapter *adapter)
 {
 	int i;
 
+	if (!adapter->tx_rings)
+		return;
+
 	for (i = 0; i < adapter->num_active_queues; i++)
 		if (adapter->tx_rings[i].desc)
 			i40evf_free_tx_resources(&adapter->tx_rings[i]);
@@ -2102,6 +2105,9 @@ void i40evf_free_all_rx_resources(struct i40evf_adapter *adapter)
 {
 	int i;
 
+	if (!adapter->rx_rings)
+		return;
+
 	for (i = 0; i < adapter->num_active_queues; i++)
 		if (adapter->rx_rings[i].desc)
 			i40evf_free_rx_resources(&adapter->rx_rings[i]);