[S390] signal: use set_restore_sigmask() helper

We should call set_restore_sigmask() instead of directly setting
TIF_RESTORE_SIGMASK. This change should have been done three years
earlier... see 4e4c22 "signals: add set_restore_sigmask".
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>

arch/s390/kernel/signal.c

@@ -65,7 +65,7 @@ SYSCALL_DEFINE3(sigsuspend, int, history0, int, history1, old_sigset_t, mask)
 	set_current_blocked(&blocked);
 	set_current_state(TASK_INTERRUPTIBLE);
 	schedule();
-	set_thread_flag(TIF_RESTORE_SIGMASK);
+	set_restore_sigmask();
 	return -ERESTARTNOHAND;
 }