s390/mm/ptdump: Add support for relocated lowcore mapping

The page table dumper contains a hard coded assumption that the first
mapped area starts at address zero. With a relocated lowcore this is
not true anymore. Subsequently the first entry (lowcore) is printed as
if it would contain everything from address zero until the end of the
location of the lowcore area.

Fix this by adding a single "Kernel Virtual Address Space" entry,
which always starts at address zero. It ends when the lowcore area
starts which is either address zero, or its relocated address.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>