drm/armada: don't grab dev->struct_mutex for in mmap offset ioctl

Since David Herrmann's mmap vma manager rework we don't need to grab
dev->struct_mutex any more to prevent races when looking up the mmap
offset. Drop it and instead don't forget to use the unref_unlocked
variant (since the drm core still cares).

v2: Split out the leak fix in dump_map_offset into a separate patch as
requested by Russell. Also align labels the same way as before to
stick with local coding style.

Signed-off-by: Daniel Vetter <[e-mail]>
Signed-off-by: Russell King <[e-mail]>

drivers/gpu/drm/armada/armada_gem.c

···
 	struct armada_gem_object *obj;
 	int ret = 0;
 
-	mutex_lock(&dev->struct_mutex);
 	obj = armada_gem_object_lookup(dev, file, handle);
 	if (!obj) {
 		DRM_ERROR("failed to lookup gem object\n");
-		ret = -EINVAL;
-		goto err_unlock;
+		return -EINVAL;
 	}
 
 	/* Don't allow imported objects to be mapped */
···
 	}
 
  err_unref:
-	drm_gem_object_unreference(&obj->obj);
- err_unlock:
-	mutex_unlock(&dev->struct_mutex);
+	drm_gem_object_unreference_unlocked(&obj->obj);
 
 	return ret;
 }