io_uring: Fix unused function warnings

If CONFIG_NET is not set, gcc warns:

fs/io_uring.c:3110:12: warning: io_setup_async_msg defined but not used [-Wunused-function]
 static int io_setup_async_msg(struct io_kiocb *req,
            ^~~~~~~~~~~~~~~~~~

There are many funcions wraped by CONFIG_NET, move them
together to simplify code, also fix this warning.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>

Minor tweaks.

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