dma-buf: system_heap: Use 'for_each_sgtable_sg' in pages free flow

For previous version, it uses 'sg_table.nent's to traverse sg_table in pages
free flow.
However, 'sg_table.nents' is reassigned in 'dma_map_sg', it means the number of
created entries in the DMA adderess space.
So, use 'sg_table.nents' in pages free flow will case some pages can't be freed.

Here we should use sg_table.orig_nents to free pages memory, but use the
sgtable helper 'for each_sgtable_sg'(, instead of the previous rather common
helper 'for_each_sg' which maybe cause memory leak) is much better.

Fixes: d963ab0f15fb0 ("dma-buf: system_heap: Allocate higher order pages if available")
Signed-off-by: Guangming <[e-mail]>
Reviewed-by: Robin Murphy <[e-mail]>
Cc: <[e-mail]> # 5.11.*
Reviewed-by: Christian König <[e-mail]>
Reviewed-by: John Stultz <[e-mail]>
Signed-off-by: Sumit Semwal <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/dma-buf/heaps/system_heap.c

···
 	int i;
 
 	table = &buffer->sg_table;
-	for_each_sg(table->sgl, sg, table->nents, i) {
+	for_each_sgtable_sg(table, sg, i) {
 		struct page *page = sg_page(sg);
 
 		__free_pages(page, compound_order(page));