powerpc: Handle simultaneous interrupts at once

It often happens to have simultaneous interrupts, for instance
when having double Ethernet attachment. With the current
implementation, we suffer the cost of kernel entry/exit for each
interrupt.

This patch introduces a loop in __do_irq() to handle all interrupts
at once before returning.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/irq.c

@@ -481,8 +481,12 @@ void __do_irq(struct pt_regs *regs)
 	if (unlikely(!irq))
 		__this_cpu_inc(irq_stat.spurious_irqs);
 	else
+		do {
 			generic_handle_irq(irq);
 
+			irq = ppc_md.get_irq();
+		} while (irq);
+
 	trace_irq_exit(regs);
 
 	irq_exit();