HWPOISON, hugetlb: fix unpoison for hugepage

Currently unpoisoning hugepages doesn't work correctly because
clearing PG_HWPoison is done outside if (TestClearPageHWPoison).
This patch fixes it.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andi Kleen <ak@linux.intel.com>