bcache: add io error counting in write_bdev_super_endio()

When backing device super block is written by bch_write_bdev_super(),
the bio complete callback write_bdev_super_endio() simply ignores I/O
status. Indeed such write request also contribute to backing device
health status if the request failed.

This patch checkes bio->bi_status in write_bdev_super_endio(), if there
is error, bch_count_backing_io_errors() will be called to count an I/O
error to dc->io_errors.
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/super.c

@@ -197,7 +197,9 @@ static const char *read_super(struct cache_sb *sb, struct block_device *bdev,
 static void write_bdev_super_endio(struct bio *bio)
 {
 	struct cached_dev *dc = bio->bi_private;
-	/* XXX: error checking */
+
+	if (bio->bi_status)
+		bch_count_backing_io_errors(dc, bio);
 
 	closure_put(&dc->sb_write);
 }