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

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 <yinghai@kernel.org>
Acked-by: Suresh Siddha <suresh.b.siddha@intel.com>
LKML-Reference: <4A785AFF.3050902@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>