net/9p: add __init/__exit annotations to module init/exit funcs

xen transport was missing annotations

Link: https://lkml.kernel.org/r/20220909103546.73015-1-xiujianfeng@huawei.com


Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>