io_uring: code clean for some ctx usage

There are some functions doing ctx = req->ctx while still using
req->ctx, update those places.

Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
Link: https://lore.kernel.org/r/20211214055904.61772-1-haoxu@linux.alibaba.com


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