brd: fix overflow in __brd_direct_access

The code in __brd_direct_access multiplies the pgoff variable by page size
and divides it by 512. It can cause overflow on 32-bit architectures. The
overflow happens if we create ramdisk larger than 4G and use it as a
sparse device.

This patch replaces multiplication and division with multiplication by the
number of sectors per page.
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: 1647b9b9 ("brd: add dax_operations support")
Cc: stable@vger.kernel.org	# 4.12+
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/brd.c

@@ -342,7 +342,7 @@ static long __brd_direct_access(struct brd_device *brd, pgoff_t pgoff,
 
 	if (!brd)
 		return -ENODEV;
-	page = brd_insert_page(brd, PFN_PHYS(pgoff) / 512);
+	page = brd_insert_page(brd, (sector_t)pgoff << PAGE_SECTORS_SHIFT);
 	if (!page)
 		return -ENOSPC;
 	*kaddr = page_address(page);