HWPOISON, hugetlb: support hwpoison injection for hugepage

This patch enables hwpoison injection through debug/hwpoison interfaces,
with which we can test memory error handling for free or reserved
hugepages (which cannot be tested by madvise() injector).

[AK: Export PageHuge too for the injection module]
Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>