HWPOISON: fix wrong num_poisoned_pages in handling memory error on thp

num_poisoned_pages counts up the number of pages isolated by memory errors.
But for thp, only one subpage is isolated because memory error handler
splits it, so it's wrong to add (1 << compound_trans_order).

ChangeLog v2:
 - commented more.
 - used compound_order instead of compound_trans_order.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>