include/linux/pid.h: use for_each_thread() in do_each_pid_thread()

while_each_pid_thread() is using while_each_thread(), which is unsafe
under RCU lock according to commit 0c740d0 ("introduce
for_each_thread() to replace the buggy while_each_thread()").  Use
for_each_thread() in do_each_pid_thread() which is safe under RCU lock.

Link: http://lkml.kernel.org/r/201702011947.DBD56740.OMVHOLOtSJFFFQ@I-love.SAKURA.ne.jp
Link: http://lkml.kernel.org/r/1486041779-4401-2-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/pid.h

@@ -191,10 +191,10 @@ pid_t pid_vnr(struct pid *pid);
 #define do_each_pid_thread(pid, type, task)				\
 	do_each_pid_task(pid, type, task) {				\
 		struct task_struct *tg___ = task;			\
-		do {
+		for_each_thread(tg___, task) {
 
 #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 */