io_uring: Fix getting file for non-fd opcodes

For timeout requests and bunch of others io_uring tries to grab a file
with specified fd, which is usually stdin/fd=0.
Update io_op_needs_file()

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>