pid namespaces: allow signalling cgroup-init

Only the global-init process must be special - any other cgroup-init
process must be killable to prevent run-away processes in the system.

TODO: 	Ideally we should allow killing the cgroup-init only from parent
	cgroup and prevent it being killed from within the cgroup.
	But that is a more complex change and will be addressed by a follow-on
	patch. For now allow the cgroup-init to be terminated by any process
	with sufficient privileges.

Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Acked-by: Pavel Emelyanov <xemul@openvz.org>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Cc: Paul Menage <menage@google.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/signal.c

@@ -1835,11 +1835,9 @@ relock:
 			continue;
 
 		/*
-		 * Init of a pid space gets no signals it doesn't want from
-		 * within that pid space. It can of course get signals from
-		 * its parent pid space.
+		 * Global init gets no signals it doesn't want.
 		 */
-		if (current == task_child_reaper(current))
+		if (is_global_init(current))
 			continue;
 
 		if (sig_kernel_stop(signr)) {