[S390] VMEM_MAX_PHYS overflow on 31 bit.

With the new space saving spinlock_t and a non-debug configuration
the struct page only has 32 bytes for 31 bit s390. The causes an
overflow in the calculation of VMEM_MAX_PHYS which renders the
kernel unbootable.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>