io_uring: ensure io-wq context is always destroyed for tasks

If the task ends up doing no IO, the context list is empty and we don't
call into __io_uring_files_cancel() when the task exits. This can cause
a leak of the io-wq structures.

Ensure we always call __io_uring_files_cancel(), even if the task
context list is empty.

Fixes: 5aa75ed5

 ("io_uring: tie async worker side to the task context")
Signed-off-by: Jens Axboe <axboe@kernel.dk>