s390/smp: enforce lowcore protection on CPU restart

As result of commit 915fea04

 ("s390/smp: enable DAT before
CPU restart callback is called") the low-address protection bit
gets mistakenly unset in control register 0 save area of the
absolute zero memory. That area is used when manual PSW restart
happened to hit an offline CPU. In this case the low-address
protection for that CPU will be dropped.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Fixes: 915fea04

 ("s390/smp: enable DAT before CPU restart callback is called")
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>