arm64: mm: tidy up top of kernel VA space

arm64: mm: tidy up top of kernel VA space

Tidy up the way the top of the kernel VA space is organized, by mirroring
the 256 MB region we have below the vmalloc space, and populating it top
down with the PCI I/O space, some guard regions, and the fixmap region.
The latter region is itself populated top down, and today only covers
about 4 MB, and so 224 MB is ample, and no guard region is therefore
required.

The resulting layout is identical between 48-bit/4k and 52-bit/64k
configurations.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Steve Capper <steve.capper@arm.com>
Link: https://lore.kernel.org/r/20201008153602.9467-5-ardb@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>