KVM: x86: Skip IOMMU IRTE updates if there's no old or new vCPU being targeted

Don't "reconfigure" an IRTE into host controlled mode when it's already in
the state, i.e. if KVM's GSI routing changes but the IRQ wasn't and still
isn't being posted to a vCPU.

Link: https://lore.kernel.org/r/20250611224604.313496-37-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/irq.c

@@ -542,6 +542,9 @@ static int kvm_pi_update_irte(struct kvm_kernel_irqfd *irqfd,
 			vcpu = NULL;
 	}
 
+	if (!irqfd->irq_bypass_vcpu && !vcpu)
+		return 0;
+
 	r = kvm_x86_call(pi_update_irte)(irqfd, irqfd->kvm, host_irq, irqfd->gsi,
 					 vcpu, irq.vector);
 	if (r) {