Total 16 bytes can be saved in two ways:

1) The field 'bio' will only be used in bio based mode, and the field
   'rq' will only be used in mq mode. Since they won't be used in the
   same time, declare a union for them.
2) The field 'bool fake_timeout' can be placed in the hole after the
   field 'error'.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20220426022133.3999006-1-yukuai3@huawei.comSigned-off-by: Jens Axboe <axboe@kernel.dk>