io_uring: move provided and fixed buffers into the same io_kiocb area

These are mutually exclusive - if you use provided buffers, then you
cannot use fixed buffers and vice versa. Move them into the same spot
in the io_kiocb, which is also advantageous for provided buffers as
they get near the submit side hot cacheline.

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