ixgbe: fix ethtool loopback diagnostic with DCB enabled

This patch disables DCB prior to running the loopback test.
When DCB is enabled the frames may be modified on Tx (by adding vlan tag)
which will fail the check on Rx.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Tested-by: Jack Morgan <jack.morgan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/ixgbe/ixgbe_ethtool.c

@@ -1805,6 +1805,10 @@ static int ixgbe_run_loopback_test(struct ixgbe_adapter *adapter)
 	unsigned int size = 1024;
 	netdev_tx_t tx_ret_val;
 	struct sk_buff *skb;
+	u32 flags_orig = adapter->flags;
+
+	/* DCB can modify the frames on Tx */
+	adapter->flags &= ~IXGBE_FLAG_DCB_ENABLED;
 
 	/* allocate test skb */
 	skb = alloc_skb(size, GFP_KERNEL);
@@ -1857,6 +1861,7 @@ static int ixgbe_run_loopback_test(struct ixgbe_adapter *adapter)
 
 	/* free the original skb */
 	kfree_skb(skb);
+	adapter->flags = flags_orig;
 
 	return ret_val;
 }