io_uring: fix off-by-one in sync cancelation file check

The passed in index should be validated against the number of registered
files we have, it needs to be smaller than the index value to avoid going
one beyond the end.

Fixes: 78a861b9 ("io_uring: add sync cancelation API through io_uring_register()")
Reported-by: Luo Likang <luolikang@nsfocus.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>