KVM: arm64: Make fault_ipa immutable

KVM: arm64: Make fault_ipa immutable

Updating fault_ipa is conceptually annoying, as it changes something
that is a property of the fault itself.

Stop doing so and instead use fault->gfn as the sole piece of state
that can be used to represent the faulting IPA.

At the same time, introduce get_canonical_gfn() for the couple of case
we're we are concerned with the memslot-related IPA and not the faulting
one.

Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>