exec: Move cleanup of posix timers on exec out of de_thread

These functions have very little to do with de_thread move them out
of de_thread an into flush_old_exec proper so it can be more clearly
seen what flush_old_exec is doing.

Signed-off-by: "Eric W. Biederman" <[e-mail]>
Reviewed-by: Bernd Edlinger <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Acked-by: Christian Brauner <[e-mail]>
Reviewed-by: Kirill Tkhai <[e-mail]>
Signed-off-by: Bernd Edlinger <[e-mail]>
Signed-off-by: Eric W. Biederman <[e-mail]>

fs/exec.c

···
 	/* we have changed execution domain */
 	tsk->exit_signal = SIGCHLD;
 
-#ifdef CONFIG_POSIX_TIMERS
-	exit_itimers(sig);
-	flush_itimer_signals();
-#endif
-
 	BUG_ON(!thread_group_leader(tsk));
 	return 0;
 
···
 	retval = de_thread(me);
 	if (retval)
 		goto out;
+
+#ifdef CONFIG_POSIX_TIMERS
+	exit_itimers(me->signal);
+	flush_itimer_signals();
+#endif
 
 	/*
 	 * Make the signal table private.