drm/amdgpu: fix userptr BO unpin bug (v2)

sg could point to array of contigiouse page*, only free page could lead
to memory leak.

v2: use iterator
Signed-off-by: Nmonk.liu <monk.liu@amd.com>
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c

@@ -555,8 +555,7 @@ static void amdgpu_ttm_tt_unpin_userptr(struct ttm_tt *ttm)
 {
 	struct amdgpu_device *adev = amdgpu_get_adev(ttm->bdev);
 	struct amdgpu_ttm_tt *gtt = (void *)ttm;
-	struct scatterlist *sg;
-	int i;
+	struct sg_page_iter sg_iter;
 
 	int write = !(gtt->userflags & AMDGPU_GEM_USERPTR_READONLY);
 	enum dma_data_direction direction = write ?
@@ -569,9 +568,8 @@ static void amdgpu_ttm_tt_unpin_userptr(struct ttm_tt *ttm)
 	/* free the sg table and pages again */
 	dma_unmap_sg(adev->dev, ttm->sg->sgl, ttm->sg->nents, direction);
 
-	for_each_sg(ttm->sg->sgl, sg, ttm->sg->nents, i) {
-		struct page *page = sg_page(sg);
-
+	for_each_sg_page(ttm->sg->sgl, &sg_iter, ttm->sg->nents, 0) {
+		struct page *page = sg_page_iter_page(&sg_iter);
 		if (!(gtt->userflags & AMDGPU_GEM_USERPTR_READONLY))
 			set_page_dirty(page);