KVM: arm64: Don't let userspace write to PMCR_EL0.N when the vcpu has EL2

Now that userspace can provide its limit for hte maximum number of
counters, prevent it from writing to PMCR_EL0.N, as the value should
be derived from MDCR_EL2.HPMN in that case.

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

arch/arm64/kvm/sys_regs.c

@@ -1216,6 +1216,7 @@ static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
 	 * with the existing KVM behavior.
 	 */
 	if (!kvm_vm_has_ran_once(kvm) &&
+	    !vcpu_has_nv(vcpu)	      &&
 	    new_n <= kvm_arm_pmu_get_max_counters(kvm))
 		kvm->arch.nr_pmu_counters = new_n;