s390/boot: Reduce size of identity mapping on overlap

In case vmemmap array could overlap with vmalloc area on
virtual memory layout setup, the size of vmalloc area
is decreased. That could result in less memory than user
requested with vmalloc= kernel command line parameter.
Instead, reduce the size of identity mapping (and the
size of vmemmap array as result) to avoid such overlap.

Further, currently the virtual memmory allocation "rolls"
from top to bottom and it is only VMALLOC_START that could
get increased due to the overlap. Change that to decrease-
only, which makes the whole allocation algorithm more easy
to comprehend.

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