ixgbevf: Fix panic when loading driver

This patch addresses a kernel panic seen when setting up the interface.
Specifically we see a NULL pointer dereference on the Tx descriptor cleanup
path when enabling interrupts.  This change corrects that so it cannot
occur.
Signed-off-by: NAlexander Duyck <alexander.h.duyck@intel.com>
Acked-by: NGreg Rose <gregory.v.rose@intel.com>
Tested-by: NSibai Li <sibai.li@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c

@@ -201,6 +201,9 @@ static bool ixgbevf_clean_tx_irq(struct ixgbevf_adapter *adapter,
 	unsigned int i, eop, count = 0;
 	unsigned int total_bytes = 0, total_packets = 0;
 
+	if (test_bit(__IXGBEVF_DOWN, &adapter->state))
+		return true;
+
 	i = tx_ring->next_to_clean;
 	eop = tx_ring->tx_buffer_info[i].next_to_watch;
 	eop_desc = IXGBE_TX_DESC_ADV(*tx_ring, eop);