x86, ioapic: Clean up ioapic/apic_id usage

While looking at the code, apic_id sometime is referred to index
of ioapic, but sometime is used for phys apic id. and some even
use apic for real apic id. It is very confusing.

So try to limit apic_id or ioapic_id to be real apic id for
ioapic, and use ioapic_idx for ioapic index in the array.

-v2: Suggested by Ingo, use ioapic_idx consistently, instead of ioapic

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Naga Chumbalkar <nagananda.chumbalkar@hp.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Link: http://lkml.kernel.org/r/4E9542DC.3090509@oracle.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>