[S390] Fix memory detection.

Yet another patch in the countless series of memory detection fixes:
if the last area of the reported storage size is a hole the detection
loop will loop forever.
Just break chunk detection loop if its end is going to be larger than
reported storage size.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>