drm/ttm: ttm object security fixes for render nodes

When a client looks up a ttm object, don't look it up through the device hash
table, but rather from the file hash table. That makes sure that the client
has indeed put a reference on the object, or in gem terms, has opened
the object; either using prime or using the global "name".

To avoid a performance loss, make sure the file hash table entries can be
looked up from under an RCU lock, and as a consequence, replace the rwlock
with a spinlock, since we never need to take it in read mode only anymore.

Finally add a ttm object lookup function for the device hash table, that is
intended to be used when we put a ref object on a base object or, in  gem terms,
when we open the object.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

drivers/gpu/drm/vmwgfx/vmwgfx_fence.c

@@ -1080,7 +1080,8 @@ int vmw_fence_event_ioctl(struct drm_device *dev, void *data,
 	 */
 	if (arg->handle) {
 		struct ttm_base_object *base =
-			ttm_base_object_lookup(vmw_fp->tfile, arg->handle);
+			ttm_base_object_lookup_for_ref(dev_priv->tdev,
+						       arg->handle);
 
 		if (unlikely(base == NULL)) {
 			DRM_ERROR("Fence event invalid fence object handle "