io_uring: fix provided buffer return on failure for kiocb_done()

Use io_req_complete_failed() in kiocb_done(). This cleans up the code,
but also ensures that a provided buffers is correctly freed on failure.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/a4880106fcf199d5810707fe2d17126fcdf18bc4.1647481208.git.asml.silence@gmail.com


[axboe: split from previous patch]
Signed-off-by: Jens Axboe <axboe@kernel.dk>