io_uring: simplify io_file_supports_nowait()

Make sure that REQ_F_SUPPORT_NOWAIT is always set io_prep_rw(), and so
we can stop caring about setting it down the line simplifying
io_file_supports_nowait().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/60c8f1f5e2cb45e00f4897b2cec10c5b3669da91.1634425438.git.asml.silence@gmail.com


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