i40e: rework exit flow of i40e_add_fdir_ethtool

Refactor the exit flow of the i40e_add_fdir_ethtool function. Move the
input_label to the end of the function, removing the dependency on
having a non-zero return value. Add a comment explaining why it is ok
not to free the fdir data structure, because the structure is now stored
in the fdir_filter_list.

Change-Id: I723342181d59cd0c9f3b31140c37961ba37bb242
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/i40e/i40e_ethtool.c

@@ -2828,12 +2828,19 @@ static int i40e_add_fdir_ethtool(struct i40e_vsi *vsi,
 	}
 
 	ret = i40e_add_del_fdir(vsi, input, true);
-free_input:
 	if (ret)
-		kfree(input);
-	else
+		goto free_input;
+
+	/* Add the input filter to the fdir_input_list, possibly replacing
+	 * a previous filter. Do not free the input structure after adding it
+	 * to the list as this would cause a use-after-free bug.
+	 */
 	i40e_update_ethtool_fdir_entry(vsi, input, fsp->location, NULL);
 
+	return 0;
+
+free_input:
+	kfree(input);
 	return ret;
 }