This moves the ksoftirqd pointers out of the irq_stat struct, and uses a
normal per-cpu variable.  It's not that time critical, nor referenced in
assembler.  This moves us closer to making irq_stat a per-cpu variable.

Because some archs have hardcoded asm references to offsets in this
structure, I haven't touched non-x86.  The __ksoftirqd_task field is
unused in other archs, too.