kvm: optimize out smp_mb after srcu_read_unlock

I noticed that srcu_read_lock/unlock both have a memory barrier,
so just by moving srcu_read_unlock earlier we can get rid of
one call to smp_mb() using smp_mb__after_srcu_read_unlock instead.

Unsurprisingly, the gain is small but measureable using the unit test
microbenchmark:
before
        vmcall in the ballpark of 1410 cycles
after
        vmcall in the ballpark of 1360 cycles

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>

arch/x86/kvm/x86.c

@@ -5966,10 +5966,12 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 
 	vcpu->mode = IN_GUEST_MODE;
 
+	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
+
 	/* We should set ->mode before check ->requests,
 	 * see the comment in make_all_cpus_request.
 	 */
-	smp_mb();
+	smp_mb__after_srcu_read_unlock();
 
 	local_irq_disable();
 
@@ -5979,12 +5981,11 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 		smp_wmb();
 		local_irq_enable();
 		preempt_enable();
+		vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
 		r = 1;
 		goto cancel_injection;
 	}
 
-	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
-
 	if (req_immediate_exit)
 		smp_send_reschedule(vcpu->cpu);