At the moment, there are two different switches used, irqbalance_disabled
and no_balance_irq ... each of which switches half the code off. This
patch harmonises them into one (irqbalance_disable), and uses the old
subarch stuff as a default value so that this is auto-disabled on boxes
like NUMA-Q that can't cope with it.

Also renamed no_balance_irq to NO_BALANCE_IRQ as it's really a static
defined number now, not pretending to be a switch variable any more.
Now off by default for NUMA-Q, on by default for others, but can be
disabled with the boot time flag if people desire.