direct-io: be sure to assign dio->bio_bdev for both paths

btrfs sets ->submit_io(), and we failed to set the block dev for
that path. That resulted in a potential NULL dereference when
we later wait for IO in dio_await_one().

Reported-by: kernel test robot <ying.huang@linux.intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>