powerpc/irq: Check desc in handle_one_irq and expand generic_handle_irq

Look up the descriptor and check that it is found in handle_one_irq
before checking if we are on the irq stack, and call the handler
directly using the descriptor if we are on the stack.

We need check irq_to_desc finds the descriptor to avoid a NULL
pointer dereference.  It could have failed because the number from
ppc_md.get_irq was above NR_IRQS, or various exceptional conditions
with sparse irqs (eg race conditions while freeing an irq if its was
not shutdown in the controller).

fe12bc2c (genirq: Uninline and sanity check generic_handle_irq())
moved generic_handle_irq out of line to allow its use by interrupt
controllers in modules.  However, handle_one_irq is core arch code.
It already knows the details of struct irq_desc and handling irqs in
the nested irq case.  This will avoid the extra stack frame to return
the value we don't check.
Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/irq.c

@@ -295,17 +295,20 @@ static inline void handle_one_irq(unsigned int irq)
 	unsigned long saved_sp_limit;
 	struct irq_desc *desc;
 
+	desc = irq_to_desc(irq);
+	if (!desc)
+		return;
+
 	/* Switch to the irq stack to handle this */
 	curtp = current_thread_info();
 	irqtp = hardirq_ctx[smp_processor_id()];
 
 	if (curtp == irqtp) {
 		/* We're already on the irq stack, just handle it */
-		generic_handle_irq(irq);
+		desc->handle_irq(irq, desc);
 		return;
 	}
 
-	desc = irq_to_desc(irq);
 	saved_sp_limit = current->thread.ksp_limit;
 
 	irqtp->task = curtp->task;