i40evf: null out ring pointers on free

BugLink: http://bugs.launchpad.net/bugs/1547674

Since we check these ring pointers to make sure we don't double-allocate
or double-free the rings, we had better null them out after we free
them. In very rare cases this can cause a panic if the driver is removed
during reset recovery.

Change-ID: Ib06eb4910a3058275c8f7ec5ef7f45baa4674f96
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 net-next commit 10311540)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

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

@@ -1122,7 +1122,9 @@ static void i40evf_free_queues(struct i40evf_adapter *adapter)
 	if (!adapter->vsi_res)
 		return;
 	kfree(adapter->tx_rings);
+	adapter->tx_rings = NULL;
 	kfree(adapter->rx_rings);
+	adapter->rx_rings = NULL;
 }
 
 /**