KVM: arm64: Convert memslot cache-flushing code to generic page-table API

Convert stage2_flush_memslot() to call the kvm_pgtable_stage2_flush()
function of the generic page-table code instead of walking the page-table
directly.

Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Cc: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200911132529.19844-16-will@kernel.org