powerpc: Copy back TIF flags on return from softirq stack

We already did it for hard IRQs but it looks like we forgot
to do it for softirqs. Without this, we would lose flags
such as TIF_NEED_RESCHED set using current_thread_info()
by something running of a softirq.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/irq.c

@@ -451,11 +451,18 @@ static inline void do_softirq_onstack(void)
 	curtp = current_thread_info();
 	irqtp = softirq_ctx[smp_processor_id()];
 	irqtp->task = curtp->task;
+	irqtp->flags = 0;
 	current->thread.ksp_limit = (unsigned long)irqtp +
 				    _ALIGN_UP(sizeof(struct thread_info), 16);
 	call_do_softirq(irqtp);
 	current->thread.ksp_limit = saved_sp_limit;
 	irqtp->task = NULL;
+
+	/* Set any flag that may have been set on the
+	 * alternate stack
+	 */
+	if (irqtp->flags)
+		set_bits(irqtp->flags, &curtp->flags);
 }
 
 void do_softirq(void)