[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>

include/asm-s390/pgtable.h

@@ -127,8 +127,9 @@ extern char empty_zero_page[PAGE_SIZE];
  * mapping. This needs to be calculated at compile time since the size of the
  * VMEM_MAP is static but the size of struct page can change.
  */
-#define VMEM_MAX_PHYS	min(VMALLOC_START, ((VMEM_MAP_END - VMALLOC_END) / \
-			  sizeof(struct page) * PAGE_SIZE) & ~((16 << 20) - 1))
+#define VMEM_MAX_PAGES	((VMEM_MAP_END - VMALLOC_END) / sizeof(struct page))
+#define VMEM_MAX_PFN	min(VMALLOC_START >> PAGE_SHIFT, VMEM_MAX_PAGES)
+#define VMEM_MAX_PHYS	((VMEM_MAX_PFN << PAGE_SHIFT) & ~((16 << 20) - 1))
 #define VMEM_MAP	((struct page *) VMALLOC_END)
 
 /*