discard bio hasn't data attached. We hit a BUG_ON with such bio. This makes
bio_split works for such bio.
Signed-off-by: NShaohua Li <shli@fusionio.com>
Signed-off-by: NNeilBrown <neilb@suse.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>