s390/mmap: increase stack/mmap gap to 128MB

This basically reverts commit 9e78a13b ("[S390] reduce miminum
gap between stack and mmap_base"). 32MB is not enough space
between stack and mmap for some programs. Given that compat
task aren't common these days, lets revert back to 128MB.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>