Fix the following two trivial bugs in pcpu_build_alloc_info()

* we should memset group_cnt to 0 by size of group_cnt, not size of
  group_map (both are of the same size, so the bug isn't dangerous)

* we can delete useless variable group_cnt_max.
Signed-off-by: NPavel V. Panteleev <pp_84@mail.ru>
Signed-off-by: NTejun Heo <tj@kernel.org>