io_uring: always initialize sqe->flags

io_uring: always initialize sqe->flags

Commit 7c70f506e438 ("engines/io_uring: move sqe clear out of hot path")
removed the memset of sqe from fio_ioring_prep().

Later, force_async was added in commit 5a59a81d2923 ("engines/io_uring:
allow setting of IOSQE_ASYNC").

The force_async commit sets sqe->flags every N requests, however,
since we no longer do a memset, this commit should have made sure that
flags is always initialized, such that we don't have sqe->flags set on
reused sqes where we didn't intend to.

Fixes: 5a59a81d2923 ("engines/io_uring: allow setting of IOSQE_ASYNC")
Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>