[S390] Rework create_mem_hole() function

This patch makes the create_mem_hole() function more readable and
fixes some minor bugs (e.g. off-by-one problems).

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>