btrfs: look at bi_size for repair decisions

bio_readpage_error currently uses bi_vcnt to decide if it is worth
retrying an I/O.  But the vector count is mostly an implementation
artifact - it really should figure out if there is more than a
single sector worth retrying.  Use bi_size for that and shift by
PAGE_SHIFT.  This really should be blocks/sectors, but given that
btrfs doesn't support a sector size different from the PAGE_SIZE
using the page size keeps the changes to a minimum.

Reviewed-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/btrfs/extent_io.c

@@ -2350,7 +2350,7 @@ static int bio_readpage_error(struct bio *failed_bio, u64 phy_offset,
 	int read_mode = 0;
 	blk_status_t status;
 	int ret;
-	unsigned failed_bio_pages = bio_pages_all(failed_bio);
+	unsigned failed_bio_pages = failed_bio->bi_iter.bi_size >> PAGE_SHIFT;
 
 	BUG_ON(bio_op(failed_bio) == REQ_OP_WRITE);