cpumask: mm_cpumask for accessing the struct mm_struct's cpu_vm_mask.

This allows us to change the representation (to a dangling bitmap or
cpumask_var_t) without breaking all the callers: they can use
mm_cpumask() now and won't see a difference as the changes roll into
linux-next.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>