wait_task_stopped(): pass correct exit_code to wait_noreap_copyout()

In wait_task_stopped() exit_code already contains the right value for the
si_status member of siginfo, and this is simply set in the non WNOWAIT
case.

If you call waitid() with a stopped or traced process, you'll get the signal
in siginfo.si_status as expected -- however if you call waitid(WNOWAIT) at the
same time, you'll get the signal << 8 | 0x7f

Pass it unchanged to wait_noreap_copyout(); we would only need to shift it
and add 0x7f if we were returning it in the user status field and that
isn't used for any function that permits WNOWAIT.

Signed-off-by: Scott James Remnant <scott@ubuntu.com>
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -1388,7 +1388,7 @@ static int wait_task_stopped(struct task_struct *p, int delayed_group_leader,
 		if (unlikely(!exit_code) || unlikely(p->exit_state))
 			goto bail_ref;
 		return wait_noreap_copyout(p, pid, uid,
-					   why, (exit_code << 8) | 0x7f,
+					   why, exit_code,
 					   infop, ru);
 	}