mount: sysfs -- Use slave mounting for the root

Seems this snippet escaeped from commit
84bf1ad4
so we may get -EBUSY in open_detach_mount.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrey Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/net.c

@@ -1390,7 +1390,7 @@ static int mount_ns_sysfs(void)
 		return -1;
 	}
 
-	if (mount(NULL, "/", NULL, MS_PRIVATE | MS_REC, NULL)) {
+	if (mount(NULL, "/", NULL, MS_SLAVE | MS_REC, NULL)) {
 		pr_perror("Can't mark the root mount as private");
 		return -1;
 	}