s390/kexec: turn DAT mode off immediately before purgatory

The kernel code is not guaranteed DAT-off mode safe.
Turn the DAT mode off immediately before entering the
purgatory.

Further, to avoid subtle side effects reset the system
immediately before turning DAT mode off while making
all necessary preparations in advance.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>