s390/smp: call smp_reinit_ipl_cpu() before scheduler is available

Currently smp_reinit_ipl_cpu() is a pre-SMP early initcall.
That ensures no CPU is running in parallel, but still not
enough to assume the code is exclusive, since the scheduling
is already available.

Move the function call to arch_call_rest_init() callback
to ensure no thread could be preempted and allow lockless
allocation of the kernel page tables. That is needed to
allow a follow-up rework of the absolute lowcore access
mechanism.

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