io_uring: remove passed in 'ctx' function parameter ctx if possible

Many times, the core of the function is req, and req has already set
req->ctx at initialization time, so there is no need to pass in the
ctx from the caller.

Cleanup, no functional change.

Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
Signed-off-by: Jens Axboe <axboe@kernel.dk>