pstree: don't replace sid if the root task is a session leader

The root task must remain a session leader after c/r. The root task
can be executed in another pidns and the sid from another name space
will be invalid.

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Reported-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

pstree.c

@@ -154,6 +154,9 @@ static int prepare_pstree_for_shell_job(void)
 	if (!opts.shell_job)
 		return 0;
 
+	if (root_item->sid == root_item->pid.virt)
+		return 0;
+
 	/*
 	 * Migration of a root task group leader is a bit tricky.
 	 * When a task yields SIGSTOP, the kernel notifies the parent