drm/ttm: Export ttm_bo_get_unless_zero()

Export ttm_bo_get_unless_zero() to be used when looking up buffer
objects that are removed from the lookup structure in the destructor.

Signed-off-by: Thomas Hellstrom <[e-mail]>
Reviewed-by: Sinclair Yeh <[e-mail]>
Reviewed-by: Christian König <[e-mail]>

drivers/gpu/drm/ttm/ttm_bo_vm.c

···
 	node = drm_vma_offset_lookup_locked(&bdev->vma_manager, offset, pages);
 	if (likely(node)) {
 		bo = container_of(node, struct ttm_buffer_object, vma_node);
-		if (!kref_get_unless_zero(&bo->kref))
-			bo = NULL;
+		bo = ttm_bo_get_unless_zero(bo);
 	}
 
 	drm_vma_offset_unlock_lookup(&bdev->vma_manager);

include/drm/ttm/ttm_bo_api.h

···
 }
 
 /**
+ * ttm_bo_get_unless_zero - reference a struct ttm_buffer_object unless
+ * its refcount has already reached zero.
+ * @bo: The buffer object.
+ *
+ * Used to reference a TTM buffer object in lookups where the object is removed
+ * from the lookup structure during the destructor and for RCU lookups.
+ *
+ * Returns: @bo if the referencing was successful, NULL otherwise.
+ */
+static inline __must_check struct ttm_buffer_object *
+ttm_bo_get_unless_zero(struct ttm_buffer_object *bo)
+{
+	if (!kref_get_unless_zero(&bo->kref))
+		return NULL;
+	return bo;
+}
+
+/**
  * ttm_bo_wait - wait for buffer idle.
  *
  * @bo:  The buffer object.