x86: Fix MSI-X initialization by using online_mask for x2apic target_cpus

found a system where x2apic reports an MSI-X irq initialization
failure:

[ 302.859446] igbvf 0000:81:10.4: enabling device (0000 -> 0002)
[ 302.874369] igbvf 0000:81:10.4: using 64bit DMA mask
[ 302.879023] igbvf 0000:81:10.4: using 64bit consistent DMA mask
[ 302.894386] igbvf 0000:81:10.4: enabling bus mastering
[ 302.898171] igbvf 0000:81:10.4: setting latency timer to 64
[ 302.914050] reserve_memtype added 0xefb08000-0xefb0c000, track uncached-minus, req uncached-minus, ret uncached-minus
[ 302.933839] reserve_memtype added 0xefb28000-0xefb29000, track uncached-minus, req uncached-minus, ret uncached-minus
[ 302.940367] alloc irq_desc for 265 on node 4
[ 302.956874] alloc kstat_irqs on node 4
[ 302.959452] alloc irq_2_iommu on node 0
[ 302.974328] igbvf 0000:81:10.4: irq 265 for MSI/MSI-X
[ 302.977778] alloc irq_desc for 266 on node 4
[ 302.980347] alloc kstat_irqs on node 4
[ 302.995312] free_memtype request 0xefb28000-0xefb29000
[ 302.998816] igbvf 0000:81:10.4: Failed to initialize MSI-X interrupts.

... it turns out that when trying to enable MSI-X,
__assign_irq_vector(new, cfg_new, apic->target_cpus()) can not
get vector because for x2apic target-cpus returns cpumask_of(0)

Update that to online_mask like xapic.

Signed-off-by: Yinghai Lu <[e-mail]>
Acked-by: Suresh Siddha <[e-mail]>
LKML-Reference: <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/kernel/apic/x2apic_cluster.c

···
 	return x2apic_enabled();
 }
 
-/* Start with all IRQs pointing to boot CPU.  IRQ balancing will shift them. */
-
+/*
+ * need to use more than cpu 0, because we need more vectors when
+ * MSI-X are used.
+ */
 static const struct cpumask *x2apic_target_cpus(void)
 {
-	return cpumask_of(0);
+	return cpu_online_mask;
 }
 
 /*

arch/x86/kernel/apic/x2apic_phys.c

···
 		return 0;
 }
 
-/* Start with all IRQs pointing to boot CPU.  IRQ balancing will shift them. */
-
+/*
+ * need to use more than cpu 0, because we need more vectors when
+ * MSI-X are used.
+ */
 static const struct cpumask *x2apic_target_cpus(void)
 {
-	return cpumask_of(0);
+	return cpu_online_mask;
 }
 
 static void x2apic_vector_allocation_domain(int cpu, struct cpumask *retmask)