s390/mm: use invalid asce for user space when switching to init_mm

Currently only idle_task_exit() explicitly switches (switch_mm) to
init_mm. This causes the kernel asce to be loaded into cr7 and
therefore it would be used for potential user space accesses.

This is currently no problem since idle_task_exit() is nearly the last
thing a CPU executes before it is taken down. However things might
change - and therefore make sure that always the invalid asce is used
for cr7 when active_mm is init_mm.

This makes sure that all potential user space accesses will fail,
instead of accessing kernel address space.

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