drm/i915/selftests: fix smatch warning in mock_reserve

If mock_region_create fails then mem will be an error pointer. Instead
we just need to use the correct ordering for the onion unwind.

igt_mock_reserve() error: 'mem' dereferencing possible ERR_PTR()
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210702104642.1189978-2-matthew.auld@intel.com

drivers/gpu/drm/i915/selftests/intel_memory_region.c

@@ -173,7 +173,7 @@ static int igt_mock_reserve(void *arg)
 	if (IS_ERR(mem)) {
 		pr_err("failed to create memory region\n");
 		err = PTR_ERR(mem);
-		goto out_close;
+		goto out_free_order;
 	}
 
 	/* Reserve a bunch of ranges within the region */
@@ -224,9 +224,10 @@ static int igt_mock_reserve(void *arg)
 	}
 
 out_close:
-	kfree(order);
 	close_objects(mem, &objects);
 	intel_memory_region_put(mem);
+out_free_order:
+	kfree(order);
 	return err;
 }