[S390] memory detection misses 128k.

Fix a memory leak problem in the memory detection routines.  A memory leak
of 128k occurs when we have a contiguous memory with mixed access-mode
(read or write) ranges.

Signed-off-by: Hongjie Yang <hongjie@us.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>