x86_64: Report the pending irq if available in smp_affinity

Otherwise smp_affinity would only update after the next interrupt
on x86 systems.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>