Isolate some explicit usage of task->tgid

With pid namespaces this field is now dangerous to use explicitly, so hide
it behind the helpers.

Also the pid and pgrp fields o task_struct and signal_struct are to be
deprecated.  Unfortunately this patch cannot be sent right now as this
leads to tons of warnings, so start isolating them, and deprecate later.

Actually the p->tgid == pid has to be changed to has_group_leader_pid(),
but Oleg pointed out that in case of posix cpu timers this is the same, and
thread_group_leader() is more preferable.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Sukadev Bhattiprolu <sukadev@us.ibm.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>
diff --git a/kernel/signal.c b/kernel/signal.c
index 783b33a..08364e7 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1150,7 +1150,7 @@
 
 		read_lock(&tasklist_lock);
 		for_each_process(p) {
-			if (p->pid > 1 && p->tgid != current->tgid) {
+			if (p->pid > 1 && !same_thread_group(p, current)) {
 				int err = group_send_sig_info(sig, info, p);
 				++count;
 				if (err != -EPERM)