KVM: arm64: np-guest CMOs with PMD_SIZE fixmap

With the introduction of stage-2 huge mappings in the pKVM hypervisor,
guest pages CMO is needed for PMD_SIZE size. Fixmap only supports
PAGE_SIZE and iterating over the huge-page is time consuming (mostly due
to TLBI on hyp_fixmap_unmap) which is a problem for EL2 latency.

Introduce a shared PMD_SIZE fixmap (hyp_fixblock_map/hyp_fixblock_unmap)
to improve guest page CMOs when stage-2 huge mappings are installed.

On a Pixel6, the iterative solution resulted in a latency of ~700us,
while the PMD_SIZE fixmap reduces it to ~100us.

Because of the horrendous private range allocation that would be
necessary, this is disabled for 64KiB pages systems.

Suggested-by: Quentin Perret <qperret@google.com>
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>
Link: https://lore.kernel.org/r/20250521124834.1070650-11-vdonnefort@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

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