vfio/spapr_tce: convert get_user_pages() --> pin_user_pages()

This code was using get_user_pages*(), in a "Case 2" scenario
(DMA/RDMA), using the categorization from [1]. That means that it's
time to convert the get_user_pages*() + put_page() calls to
pin_user_pages*() + unpin_user_pages() calls.

There is some helpful background in [2]: basically, this is a small
part of fixing a long-standing disconnect between pinning pages, and
file systems' use of those pages.

[1] Documentation/core-api/pin_user_pages.rst

[2] "Explicit pinning of user-space pages":
    https://lwn.net/Articles/807108/

Cc: Alex Williamson <[email protected]>
Cc: Cornelia Huck <[email protected]>
Cc: [email protected]
Signed-off-by: John Hubbard <[email protected]>
Signed-off-by: Alex Williamson <[email protected]>

Author: johnhubbard

drivers/vfio/vfio_iommu_spapr_tce.c

@@ -383,7 +383,7 @@ static void tce_iommu_unuse_page(struct tce_container *container,
 	struct page *page;
 
 	page = pfn_to_page(hpa >> PAGE_SHIFT);
-	put_page(page);
+	unpin_user_page(page);
 }
 
 static int tce_iommu_prereg_ua_to_hpa(struct tce_container *container,
@@ -486,7 +486,7 @@ static int tce_iommu_use_page(unsigned long tce, unsigned long *hpa)
 	struct page *page = NULL;
 	enum dma_data_direction direction = iommu_tce_direction(tce);
 
-	if (get_user_pages_fast(tce & PAGE_MASK, 1,
+	if (pin_user_pages_fast(tce & PAGE_MASK, 1,
 			direction != DMA_TO_DEVICE ? FOLL_WRITE : 0,
 			&page) != 1)
 		return -EFAULT;