[S390] use set_current_state in sigsuspend

Use set_current_state instead of a direct assignment to set the
task state of the current process.

Signed-off-by: Martin Schwidefsky <[e-mail]>

arch/s390/kernel/signal.c

···
 	recalc_sigpending();
 	spin_unlock_irq(&current->sighand->siglock);
 
-	current->state = TASK_INTERRUPTIBLE;
+	set_current_state(TASK_INTERRUPTIBLE);
 	schedule();
 	set_thread_flag(TIF_RESTORE_SIGMASK);