KVM: arm64: Defer EL2 stage-1 mapping on share

We currently blindly map into EL2 stage-1 *any* page passed to the
__pkvm_host_share_hyp() HVC. This is less than ideal from a security
perspective as it makes exploitation of potential hypervisor gadgets
easier than it should be. But interestingly, pKVM should never need to
access SHARED_BORROWED pages that it hasn't previously pinned, so there
is no need to map the page before that.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
Link: https://lore.kernel.org/r/20250416152648.2982950-7-qperret@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

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