[S390] hibernate: make sure pfn_is_nosave handles lowcore pages

pfn_is_nosave doesn't return the correct value for the second lowcore
page if lowcore protection is enabled. Make sure it always returns
the correct value.

While at it simplify the whole thing.
NSS special handling is done by the tprot check like it already works
for DCSS as well. So remove the extra code for NSS.

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