arm64: mm: Modify calculation of VMEMMAP_SIZE

In a later patch we will need to have a slightly larger VMEMMAP region
to accommodate boot time selection between 48/52-bit kernel VAs.

This patch modifies the formula for computing VMEMMAP_SIZE to depend
explicitly on the PAGE_OFFSET and start of kernel addressable memory.
(This allows for a slightly larger direct linear map in future).

Signed-off-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>