io_uring: do not use prio task_work_add in uring_cmd

io_req_task_prio_work_add has a strict assumption that it will only be
used with io_req_task_complete. There is a codepath that assumes this is
the case and will not even call the completion function if it is hit.

For uring_cmd with an arbitrary completion function change the call to the
correct non-priority version.

Fixes: ee692a21e9bf8 ("fs,io_uring: add infrastructure for uring-cmd")
Signed-off-by: Dylan Yudaken <dylany@fb.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/20220616135011.441980-1-dylany@fb.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>