sched: convert sched.c from for_each_cpu_mask to for_each_cpu.

Impact: trivial API conversion

This is a simple conversion, but note that for_each_cpu() terminates
with i >= nr_cpu_ids, not i == NR_CPUS like for_each_cpu_mask() did.

I don't convert all of them: sd->span changes in a later patch, so
change those iterators there rather than here.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>