fix setpriority(PRIO_PGRP) thread iterator breakage

When user calls sys_setpriority(PRIO_PGRP ...) on a NPTL style multi-LWP
process, only the task leader of the process is affected, all other
sibling LWP threads didn't receive the setting.  The problem was that the
iterator used in sys_setpriority() only iteartes over one task for each
process, ignoring all other sibling thread.

Introduce a new macro do_each_pid_thread / while_each_pid_thread to walk
each thread of a process.  Convert 4 call sites in {set/get}priority and
ioprio_{set/get}.

Signed-off-by: Ken Chen <kenchen@google.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/pid.h

@@ -161,4 +161,13 @@ pid_t pid_vnr(struct pid *pid);
 			}						\
 	} while (0)
 
+#define do_each_pid_thread(pid, type, task)				\
+	do_each_pid_task(pid, type, task) {				\
+		struct task_struct *tg___ = task;			\
+		do {
+
+#define while_each_pid_thread(pid, type, task)				\
+		} while_each_thread(tg___, task);			\
+		task = tg___;						\
+	} while_each_pid_task(pid, type, task)
 #endif /* _LINUX_PID_H */