s390/mm: use CRST_ALLOC_ORDER instead of number

Use CRST_ALLOC_ORDER to make it more obvious what the order means,
and also to be consistent with other code, e.g. the vmemmap code.

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