Patch from Dave Hansen <haveblue@us.ibm.com>

The new kirq patch assumes flat addressing APIC mode where apicid = (1
<< cpu).  This isn't true for clustered mode.

 - Change name/type of irq_balance_mask.  The type of apicid seems to
   be int.
 - Change instance of (1<<cpu) to cpu_to_logical_apicid()
 - Don't use target_cpu_mask, use min_loaded, and convert the real way

Tested on Summit, and plain SMP.  Martin Bligh and I figured this out
together, and he agrees.