KVM: arm64: Kill topup_memcache from kvm_s2_fault

KVM: arm64: Kill topup_memcache from kvm_s2_fault

The topup_memcache field can be easily replaced by the equivalent
conditions, and the resulting code is not much worse. While at it,
split prepare_mmu_memcache() into get/topup helpers, which makes
the code more readable.

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>