s390/mm: avoid trimming to MAX_ORDER

Trimming to MAX_ORDER was originally done in order to avoid to set
HOLES_IN_ZONE, which in turn would enable a quite expensive
pfn_valid() check. pfn_valid() however only checks if a struct page
exists for a given pfn.

With sparsemen vmemmap there are always struct pages, since memmaps
are allocated for whole sections. Therefore remove the HOLES_IN_ZONE
comment and the trimming.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>