s390/mm: rework arch_get_mappable_range() callback

As per description in mm/memory_hotplug.c platforms should define
arch_get_mappable_range() that provides maximum possible addressable
physical memory range for which the linear mapping could be created.

The current implementation uses VMEM_MAX_PHYS macro as the maximum
mappable physical address and it is simply a cast to vmemmap. Since
the address is in physical address space the natural upper limit of
MAX_PHYSMEM_BITS is honoured:

	vmemmap_start = min(vmemmap_start, 1UL << MAX_PHYSMEM_BITS);

Further, to make sure the identity mapping would not overlay with
vmemmap, the size of identity mapping could be stripped like this:

	ident_map_size = min(ident_map_size, vmemmap_start);

Similarily, any other memory that could be added (e.g DCSS segment)
should not overlay with vmemmap as well and that is prevented by
using vmemmap (VMEM_MAX_PHYS macro) as the upper limit.

However, while the use of VMEM_MAX_PHYS brings the desired result
it actually poses two issues:

1. As described, vmemmap is handled as a physical address, although
   it is actually a pointer to struct page in virtual address space.

2. As vmemmap is a virtual address it could have been located
   anywhere in the virtual address space. However, the desired
   necessity to honour MAX_PHYSMEM_BITS limit prevents that.

Rework arch_get_mappable_range() callback in a way it does not
use VMEM_MAX_PHYS macro and does not confuse the notion of virtual
vs physical address spacees as result. That paves the way for moving
vmemmap elsewhere and optimizing the virtual address space layout.

Introduce max_mappable preserved boot variable and let function
setup_kernel_memory_layout() set it up. As result, the rest of the
code is does not need to know the virtual memory layout specifics.

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