xen/9pfs: p9_trans_xen_init and p9_trans_xen_exit can be static

Fixes the following sparse warnings:

net/9p/trans_xen.c:528:5: warning:
 symbol 'p9_trans_xen_init' was not declared. Should it be static?
net/9p/trans_xen.c:540:6: warning:
 symbol 'p9_trans_xen_exit' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

net/9p/trans_xen.c

@@ -525,7 +525,7 @@ static struct xenbus_driver xen_9pfs_front_driver = {
 	.otherend_changed = xen_9pfs_front_changed,
 };
 
-int p9_trans_xen_init(void)
+static int p9_trans_xen_init(void)
 {
 	if (!xen_domain())
 		return -ENODEV;
@@ -537,7 +537,7 @@ int p9_trans_xen_init(void)
 }
 module_init(p9_trans_xen_init);
 
-void p9_trans_xen_exit(void)
+static void p9_trans_xen_exit(void)
 {
 	v9fs_unregister_trans(&p9_xen_trans);
 	return xenbus_unregister_driver(&xen_9pfs_front_driver);