io_uring: remove io_preinit_req()

Apart from setting ->ctx, io_preinit_req() zeroes a bunch of fields of a
request, from which only ->file_node is mandatory. Remove the function
and zero the entire request on first allocation. With that, we also need
to initialise ->ctx every time, which might be a good thing for
performance as now we're likely overwriting the entire cache line, and
so it can write combined and avoid RMW.

Suggested-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/ba5485dc913f1e275862ce88f5169d4ac4a33836.1746533807.git.asml.silence@gmail.com


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