drm/etnaviv: 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/Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>

drivers/gpu/drm/etnaviv/etnaviv_gem.c

@@ -675,10 +675,10 @@ static int etnaviv_gem_userptr_get_pages(struct etnaviv_gem_object *etnaviv_obj)
 		uint64_t ptr = userptr->ptr + pinned * PAGE_SIZE;
 		struct page **pages = pvec + pinned;
 
-		ret = get_user_pages_fast(ptr, num_pages,
+		ret = pin_user_pages_fast(ptr, num_pages,
 					  !userptr->ro ? FOLL_WRITE : 0, pages);
 		if (ret < 0) {
-			release_pages(pvec, pinned);
+			unpin_user_pages(pvec, pinned);
 			kvfree(pvec);
 			return ret;
 		}
@@ -702,7 +702,7 @@ static void etnaviv_gem_userptr_release(struct etnaviv_gem_object *etnaviv_obj)
 	if (etnaviv_obj->pages) {
 		int npages = etnaviv_obj->base.size >> PAGE_SHIFT;
 
-		release_pages(etnaviv_obj->pages, npages);
+		unpin_user_pages(etnaviv_obj->pages, npages);
 		kvfree(etnaviv_obj->pages);
 	}
 }