s390/mm: Create virtual memory layout structure

This is a preparatory rework to allow uncoupling virtual
and physical addresses spaces.

Put virtual memory layout information into a structure
to improve code generation when accessing the structure
members, which are currently only ident_map_size and
__kaslr_offset.

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