Impact: reduce kernel memory usage when CONFIG_CPUMASK_OFFSTACK=y

Straightforward conversion: done for 32 and 64 bit kernels.
node_to_cpumask_map is now a cpumask_var_t array.

64-bit used to be a dynamic cpumask_t array, and 32-bit used to be a
static cpumask_t array.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Impact: cleanup

We take the 64-bit code and use it on 32-bit as well.  The new file
is called mm/numa.c.

In a minor cleanup, we use cpu_none_mask instead of declaring a local
cpu_mask_none.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>