drbd: bm_page_async_io: properly initialize page->private

If bm_page_async_io is advised to use a new page for I/O
(BM_AIO_COPY_PAGES is set), it will get it from a mempool.
Once the mempool has to dip into its reserves the page is
not reinitialized, i.e. page->private contains garbage, which
will lead to various problems once the I/O completes (dereferences
of NULL pointers, the submitting thread getting stuck in D-state,
 ...).
Signed-off-by: NArne Redlich <arne.redlich@googlemail.com>
Signed-off-by: NLars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_bitmap.c

@@ -200,7 +200,7 @@ void drbd_bm_unlock(struct drbd_conf *mdev)
 static void bm_store_page_idx(struct page *page, unsigned long idx)
 {
 	BUG_ON(0 != (idx & ~BM_PAGE_IDX_MASK));
-	page_private(page) |= idx;
+	set_page_private(page, idx);
 }
 
 static unsigned long bm_page_to_idx(struct page *page)