pids: __set_special_pids: use change_pid() helper

Use change_pid() instead of detach_pid() + attach_pid() in
__set_special_pids().

This way task_session() is not NULL in between.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc:  "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -334,13 +334,11 @@ void __set_special_pids(struct pid *pid)
 	pid_t nr = pid_nr(pid);
 
 	if (task_session(curr) != pid) {
-		detach_pid(curr, PIDTYPE_SID);
-		attach_pid(curr, PIDTYPE_SID, pid);
+		change_pid(curr, PIDTYPE_SID, pid);
 		set_task_session(curr, nr);
 	}
 	if (task_pgrp(curr) != pid) {
-		detach_pid(curr, PIDTYPE_PGID);
-		attach_pid(curr, PIDTYPE_PGID, pid);
+		change_pid(curr, PIDTYPE_PGID, pid);
 		set_task_pgrp(curr, nr);
 	}
 }