Implement the set_direct_map_*() API, which allows to invalidate and set
default permissions to pages within the direct mapping.

Note that kernel_page_present(), which is also supposed to be part of this
API, is intentionally not implemented. The reason for this is that
kernel_page_present() is only used (and currently only makes sense) for
suspend/resume, which isn't supported on s390.
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>