[S390] increase default size of vmalloc area

The default size of the vmalloc area is currently 1 GB. The memory resource
controller uses about 10 MB of vmalloc space per gigabyte of memory. That
turns a system with more than ~100 GB memory unbootable with the default
vmalloc size. It costs us nothing to increase the default size to some
more adequate value, e.g. 128 GB.

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