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 0c740d0a ("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>