powerpc/xmon: simplify xmon_batch_next_cpu()

The function opencodes for_each_cpu_wrap() macro. As a loop termination
condition it uses cpumask_empty(), which is O(N), and it makes the whole
algorithm O(N^2). Switching to for_each_cpu_wrap() simplifies the logic,
and makes the algorithm linear.

Signed-off-by: Yury Norov <yury.norov@gmail.com>