KVM: arm64: vhe: Synchronise with page table walker on MMU update

Contrary to nVHE, VHE is a lot easier when it comes to dealing
with speculative page table walks started at EL1. As we only change
EL1&0 translation regime when context-switching, we already benefit
from the effect of the DSB that sits in the context switch code.

We only need to take care of it in the NV case, where we can
flip between between two EL1 contexts (one of them being the virtual
EL2) without a context switch.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>