s390/hypfs: simplify memory allocation

Simplify memory allocation for diagnose 204 memory buffer:

- allocate with __vmalloc_node() to enure page alignment
- allocate real / physical memory area also within vmalloc area and handle
  vmalloc to real / physical address translation within diag204().

Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Mete Durlu <meted@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>