s390/suspend: fix prefix register reset in swsusp_arch_resume

The reset of the prefix to zero in swsusp_arch_resume uses a 4 byte stack
slot. With CONFIG_VMAP_STACK=y this is now in the vmalloc area, this works
only with DAT enabled. Move the DAT disable in swsusp_arch_resume after
the prefix reset.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>