s390/suspend: fix stack setup in swsusp_arch_suspend

The patch that added support for the virtually mapped kernel stacks changed
swsusp_arch_suspend to switch to the nodat-stack as the vmap stack is not
available while going in and out of suspend.

Unfortunately the switch to the nodat-stack is incorrect which breaks
suspend to disk.

Cc: stable@vger.kernel.org # v4.20
Fixes: ce3dc447 ("s390: add support for virtually mapped kernel stacks")
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>