drm/ttm: never add BO that failed to validate to the LRU list

Fixes a potential race condition in amdgpu that looks as follows:

Task 1: attempt ttm_bo_init, but ttm_bo_validate fails
Task 1: add BO to global list anyway
Task 2: grabs hold of the BO, waits on its reservation lock
Task 1: releases its reference of the BO; never gives up the
        reservation lock

The patch "drm/amdgpu: fix a potential deadlock in
amdgpu_bo_create_restricted()" attempts to fix that by releasing
the reservation lock in amdgpu code; unfortunately, it introduces
a use-after-free when this race _doesn't_ happen.

This patch should fix the race properly by never adding the BO
to the global list in the first place.

Cc: zhoucm1 <david1.zhou@amd.com>
Signed-off-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Tested-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/ttm/ttm_bo.c

@@ -1188,18 +1188,20 @@ int ttm_bo_init(struct ttm_bo_device *bdev,
 	if (likely(!ret))
 		ret = ttm_bo_validate(bo, placement, interruptible, false);
 
-	if (!resv) {
+	if (!resv)
 		ttm_bo_unreserve(bo);
 
-	} else if (!(bo->mem.placement & TTM_PL_FLAG_NO_EVICT)) {
+	if (unlikely(ret)) {
+		ttm_bo_unref(&bo);
+		return ret;
+	}
+
+	if (resv && !(bo->mem.placement & TTM_PL_FLAG_NO_EVICT)) {
 		spin_lock(&bo->glob->lru_lock);
 		ttm_bo_add_to_lru(bo);
 		spin_unlock(&bo->glob->lru_lock);
 	}
 
-	if (unlikely(ret))
-		ttm_bo_unref(&bo);
-
 	return ret;
 }
 EXPORT_SYMBOL(ttm_bo_init);