s390/mm: extend default vmalloc area size to 512GB

We've seen several occurences in the past where the default vmalloc
size of 128GB is not sufficient. Therefore extend the default size.

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