io_uring: clean up io_uring_register_get_file()

Make it always reference the returned file. It's safer, especially with
unregistrations happening under it. And it makes the api cleaner with no
conditional clean ups by the caller.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/0d0b13a63e8edd6b5d360fc821dcdb035cb6b7e0.1736995897.git.asml.silence@gmail.com


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