block: don't unecessarily clobber bi_error for chained bios

Only overwrite the parents bi_error if it was zero. That way a successful
bio completion doesn't reset the error pointer.

Reported-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/bio.c

@@ -300,6 +300,7 @@ static void bio_chain_endio(struct bio *bio)
 {
 	struct bio *parent = bio->bi_private;
 
+	if (!parent->bi_error)
 		parent->bi_error = bio->bi_error;
 	bio_endio(parent);
 	bio_put(bio);
@@ -1753,6 +1754,8 @@ void bio_endio(struct bio *bio)
 		 */
 		if (bio->bi_end_io == bio_chain_endio) {
 			struct bio *parent = bio->bi_private;
+
+			if (!parent->bi_error)
 				parent->bi_error = bio->bi_error;
 			bio_put(bio);
 			bio = parent;