s390/dcssblk: Add DAX support

s390/dcssblk: Add DAX support

With ZONE_DEVICE now available for s390, struct pages can be allocated
for proper DAX support in dcssblk driver via devm_memremap_pages().

Adding struct pages for a range requires that the range is aligned to
SUBSECTION_SIZE, which is defined as 2 MB in common code. Therefore,
only enable DAX support and allocate struct pages for DCSS ranges that
are aligned to 2 MB.

Signed-off-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>