btrfs: simplify code with bio_io_error

bio_io_error was introduced in the commit 4246a0b6
("block: add a bi_error field to struct bio"), so use it to simplify
code.

Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>