KVM: arm/arm64: vgic: Use READ_ONCE fo cmpxchg

There is a small chance that the compiler could generate separate loads
for the dist->propbaser which could be modified from another CPU.  As we
want to make sure we atomically update the entire value, and don't race
with other updates, guarantee that the cmpxchg operation compares
against the original value.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Christoffer Dall <cdall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>