Impact: fix build errors

Since the SPARSE IRQS changes redefined how the kstat irqs are
organized, arch's must use the new accessor function:

	kstat_incr_irqs_this_cpu(irq, DESC);

If CONFIG_SPARSE_IRQS is set, then DESC is a pointer to the
irq_desc which has a pointer to the kstat_irqs.  If not, then
the .irqs field of struct kernel_stat is used instead.

Signed-off-by: Mike Travis <travis@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>