KVM: arm64: Upgrade VMID accesses to {READ,WRITE}_ONCE

Since TLB invalidation can run in parallel with VMID allocation,
we need to be careful and avoid any sort of load/store tearing.
Use {READ,WRITE}_ONCE consistently to avoid any surprise.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Jade Alglave <jade.alglave@arm.com>
Cc: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>
Reviewed-by: Quentin Perret <qperret@google.com>
Link: https://lore.kernel.org/r/20210806113109.2475-6-will@kernel.org