io_uring: better to use REQ_F_IO_DRAIN for req->flags

It's better to use REQ_F_IO_DRAIN for req->flags rather than
IOSQE_IO_DRAIN though they have same value.

Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
Link: https://lore.kernel.org/r/20211125092103.224502-3-haoxu@linux.alibaba.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>