s390/smp: do not use nodat_stack for secondary CPU start

s390/smp: do not use nodat_stack for secondary CPU start

The secondary CPU start C routine uses nodat_stack as a
interim stack before finally switching to kernel_stack.
Such scheme is superfluous, since the assembler restart
interrupt handler (that secondary CPU starter is called
from) does not need to use any stack for switching into
DAT mode. Once DAT is on, any stack including virtually-
mapped one could be used.

Avoid the use of nodat_stack and smp_start_secondary()
helper. Instead, initiate kernel_stack directly from
the restart interrupt handler.

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