io_uring: don't check file ops of registered rings

Registered rings are per definitions io_uring files, so we don't need to
additionally verify them.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/425cd64fd885b8e329a46c205ee811987691baaf.1656153286.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>