KVM: s390: optimize round trip time in request handling

The fast path for a sie exit is that no kvm reqest is pending.
Make an early check to skip all single bit checks.
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

arch/s390/kvm/kvm-s390.c

@@ -1720,6 +1720,8 @@ static bool ibs_enabled(struct kvm_vcpu *vcpu)
 
 static int kvm_s390_handle_requests(struct kvm_vcpu *vcpu)
 {
+	if (!vcpu->requests)
+		return 0;
 retry:
 	s390_vcpu_unblock(vcpu);
 	/*