x86-64, NUMA: make numa_cleanup_meminfo() prettier

* Factor out numa_remove_memblk_from().

* Hole detection doesn't need separate start/end.  Calculate start/end
  once.

* Relocate comment.

* Define iterators at the top and remove unnecessary prefix
  increments.

This prepares for further improvements to the function.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Shaohui Zheng <shaohui.zheng@intel.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: H. Peter Anvin <hpa@linux.intel.com>