drm/vgem: Close use-after-free race in vgem_gem_create

There's two references floating around here (for the object reference,
not the handle_count reference, that's a different thing):

- The temporary reference held by vgem_gem_create, acquired by
creating the object and released by calling
drm_gem_object_put_unlocked.

- The reference held by the object handle, created by
drm_gem_handle_create. This one generally outlives the function,
except if a 2nd thread races with a GEM_CLOSE ioctl call.

So usually everything is correct, except in that race case, where the
access to gem_object->size could be looking at freed data already.
Which again isn't a real problem (userspace shot its feet off already
with the race, we could return garbage), but maybe someone can exploit
this as an information leak.

Cc: Dan Carpenter <[e-mail]>
Cc: Hillf Danton <[e-mail]>
Reported-by: [e-mail]
Cc: [e-mail]
Cc: Emil Velikov <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Sean Paul <[e-mail]>
Cc: Chris Wilson <[e-mail]>
Cc: Eric Anholt <[e-mail]>
Cc: Sam Ravnborg <[e-mail]>
Cc: Rob Clark <[e-mail]>
Reviewed-by: Chris Wilson <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/vgem/vgem_drv.c

···
 		return ERR_CAST(obj);
 
 	ret = drm_gem_handle_create(file, &obj->base, handle);
-	drm_gem_object_put_unlocked(&obj->base);
-	if (ret)
+	if (ret) {
+		drm_gem_object_put_unlocked(&obj->base);
 		return ERR_PTR(ret);
+	}
 
 	return &obj->base;
 }
···
 	args->size = gem_object->size;
 	args->pitch = pitch;
 
-	DRM_DEBUG("Created object of size %lld\n", size);
+	drm_gem_object_put_unlocked(gem_object);
+
+	DRM_DEBUG("Created object of size %llu\n", args->size);
 
 	return 0;
 }