On i386, kernel irq balance doesn't work.

1) In function do_irq_balance, after kernel finds the min_loaded cpu but
   before calling set_pending_irq to really pin the selected_irq to the
   target cpu, kernel does a cpus_and with irq_affinity[selected_irq].
   Later on, when the irq is acked, kernel would calls
   move_native_irq=>desc->handler->set_affinity to change the irq affinity.
    However, every function pointed by
   hw_interrupt_type->set_affinity(unsigned int irq, cpumask_t cpumask)
   always changes irq_affinity[irq] to cpumask.  Next time when recalling
   do_irq_balance, it has to do cpu_ands again with
   irq_affinity[selected_irq], but irq_affinity[selected_irq] already
   becomes one cpu selected by the first irq balance.

2) Function balance_irq in file arch/i386/kernel/io_apic.c has the same
   issue.

[akpm@osdl.org: cleanups]
Signed-off-by: Zhang Yanmin <yanmin.zhang@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>