powerpc: Fix irq distribution

setting CONFIG_IRQ_ALL_CPUS distributes IRQs to CPUs only when
the number of online CPUs equals NR_CPUS.  See commit
280ff974 "sparc64: fix and
optimize irq distribution" for more details.

Using the online mask fixes IRQ-to-CPU distribution on systems
that boot with less than NR_CPUS.
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/irq.c

@@ -587,7 +587,7 @@ int irq_choose_cpu(const struct cpumask *mask)
 {
 	int cpuid;
 
-	if (cpumask_equal(mask, cpu_all_mask)) {
+	if (cpumask_equal(mask, cpu_online_mask)) {
 		static int irq_rover;
 		static DEFINE_RAW_SPINLOCK(irq_rover_lock);
 		unsigned long flags;