KVM: arm64: Rework kvm_pgtable initialisation

Ganapatrao reported that the kvm_pgtable->mmu pointer is more or
less hardcoded to the main S2 mmu structure, while the nested
code needs it to point to other instances (as we have one instance
per nested context).

Rework the initialisation of the kvm_pgtable structure so that
this assumtion doesn't hold true anymore. This requires some
minor changes to the order in which things are initialised
(the mmu->arch pointer being the critical one).

Reported-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
Reviewed-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211129200150.351436-5-maz@kernel.org