cpumask: complete CONFIG_CPUMASK_OFFSTACK

This patch series finally gets the last remaining cpumask_t in the core kernel, and thus allows us to make 'struct cpumask' undefined in the CONFIG_CPUMASK_OFFSTACK case. This gives us confidence that noone is manipulating a cpumask except via the cpumask_ functions, so we can finally short-allocate them (ie. nr_cpu_ids not NR_CPUS) for the CONFIG_CPUMASK_OFFSTACK=y case.

This series has been tested on 32-bit only, so handle with care!

Cheers,
Rusty.