cpumask: remove cpumask_t from core

Impact: cleanup

struct cpumask is nicer, and we use it to make where we've made code
safe for CONFIG_CPUMASK_OFFSTACK=y.

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

diff --cc kernel/sched_cpupri.h
Simple merge

diff --cc kernel/stop_machine.c
Simple merge