KVM: arm64: Don't pass kvm_pgtable through kvm_pgtable_walk_data

In order to tear down page tables from outside the context of
kvm_pgtable (such as an RCU callback), stop passing a pointer through
kvm_pgtable_walk_data.

No functional change intended.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Reviewed-by: Ben Gardon <bgardon@google.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221107215644.1895162-5-oliver.upton@linux.dev