dm: use bdev_nr_sectors and bdev_nr_bytes instead of open coding them

Use the proper helpers to read the block device size.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Snitzer <snitzer@redhat.com>
Link: https://lore.kernel.org/r/20211018101130.1838532-6-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/dm-writecache.c

@@ -2341,7 +2341,7 @@ static int writecache_ctr(struct dm_target *ti, unsigned argc, char **argv)
 		ti->error = "Cache data device lookup failed";
 		goto bad;
 	}
-	wc->memory_map_size = i_size_read(wc->ssd_dev->bdev->bd_inode);
+	wc->memory_map_size = bdev_nr_bytes(wc->ssd_dev->bdev);
 
 	/*
 	 * Parse the cache block size