Patch from "Martin J. Bligh" <mbligh@aracnet.com>

Fixes up the logic and code which is used to suppress the IRQ balancing code.

We now just have a single boolean, "irqbalance_disabled".  The initial value
comes from the per-platform "NO_BALANCE_IRQ" constant.

If the platform defaults to "on", users can override this with the
"noirqbalance" kernel boot option.