dm writecache: use bdev_nr_sectors() instead of open-coded equivalent

Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-writecache.c

@@ -978,7 +978,7 @@ static void writecache_resume(struct dm_target *ti)
 
 	wc_lock(wc);
 
-	wc->data_device_sectors = i_size_read(wc->dev->bdev->bd_inode) >> SECTOR_SHIFT;
+	wc->data_device_sectors = bdev_nr_sectors(wc->dev->bdev);
 
 	if (WC_MODE_PMEM(wc)) {
 		persistent_memory_invalidate_cache(wc->memory_map, wc->memory_map_size);