io_uring: fix submission-failure handling for uring-cmd

If ->uring_cmd returned an error value different from -EAGAIN or
-EIOCBQUEUED, it gets overridden with IOU_OK. This invites trouble
as caller (io_uring core code) handles IOU_OK differently than other
error codes.
Fix this by returning the actual error code.

Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>