dm writecache: remove unused member page_offset in writeback_struct

The stucture member page_offset in writeback_struct never has been
used actually. Remove it.
Signed-off-by: Huaisheng Ye <yehs1@lenovo.com>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-writecache.c

@@ -190,7 +190,6 @@ struct writeback_struct {
 	struct dm_writecache *wc;
 	struct wc_entry **wc_list;
 	unsigned wc_list_n;
-	unsigned page_offset;
 	struct page *page;
 	struct wc_entry *wc_list_inline[WB_LIST_INLINE];
 	struct bio bio;
@@ -1481,7 +1480,6 @@ static void __writecache_writeback_pmem(struct dm_writecache *wc, struct writeba
 		bio->bi_end_io = writecache_writeback_endio;
 		bio_set_dev(bio, wc->dev->bdev);
 		bio->bi_iter.bi_sector = read_original_sector(wc, e);
-		wb->page_offset = PAGE_SIZE;
 		if (max_pages <= WB_LIST_INLINE ||
 		    unlikely(!(wb->wc_list = kmalloc_array(max_pages, sizeof(struct wc_entry *),
 							   GFP_NOIO | __GFP_NORETRY |