s390/hugetlb: use direct TLB flushing for hugetlbfs pages

huge_ptep_get_and_clear() is either missing a TLB invalidation or
an mm->context.attach_count check. Since the attach_count logic was
introduced with normal ptes in mind, let's just use direct TLB
flushing for hugetlbfs pages.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>