io_uring: remove req->needs_fixed_files

A submission is "async" IIF it's done by SQPOLL thread. Instead of
passing @async flag into io_submit_sqes(), deduce it from ctx->flags.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>