KVM: arm64: timers: Use a per-vcpu, per-timer accumulator for fractional ns

Instead of accumulating the fractional ns value generated every time
we compute a ns delta in a global variable, use a per-vcpu, per-timer
variable. This keeps the fractional ns local to the timer instead of
contributing to any odd, unrelated timer.

Reviewed-by: Colton Lewis <coltonlewis@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230330174800.2677007-2-maz@kernel.org

arch/arm64/kvm/arch_timer.c

@@ -212,7 +212,7 @@ static u64 kvm_counter_compute_delta(struct arch_timer_context *timer_ctx,
 		ns = cyclecounter_cyc2ns(timecounter->cc,
 					 val - now,
 					 timecounter->mask,
-					 &timecounter->frac);
+					 &timer_ctx->ns_frac);
 		return ns;
 	}