[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>
diff --git a/arch/s390/kernel/signal.c b/arch/s390/kernel/signal.c
index 38a88b6..9a40e1c 100644
--- a/arch/s390/kernel/signal.c
+++ b/arch/s390/kernel/signal.c
@@ -65,7 +65,7 @@
 	set_current_blocked(&blocked);
 	set_current_state(TASK_INTERRUPTIBLE);
 	schedule();
-	set_thread_flag(TIF_RESTORE_SIGMASK);
+	set_restore_sigmask();
 	return -ERESTARTNOHAND;
 }