s390/mm: let vmalloc area size depend on physical memory size

To make sure that the vmalloc area size is for almost all cases large
enough let it depend on the (potential) physical memory size. There is
still the possibility to override this with the vmalloc kernel command
line parameter.

Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
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>