KVM: PPC: Book3S HV: Allow DTL to be set to address 0, length 0

Commit 55b665b0 ("KVM: PPC: Book3S HV: Provide a way for userspace
to get/set per-vCPU areas") includes a check on the length of the
dispatch trace log (DTL) to make sure the buffer is at least one entry
long.  This is appropriate when registering a buffer, but the
interface also allows for any existing buffer to be unregistered by
specifying a zero address.  In this case the length check is not
appropriate.  This makes the check conditional on the address being
non-zero.
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>

arch/powerpc/kvm/book3s_hv.c

@@ -811,9 +811,8 @@ int kvmppc_set_one_reg(struct kvm_vcpu *vcpu, u64 id, union kvmppc_one_reg *val)
 		addr = val->vpaval.addr;
 		len = val->vpaval.length;
 		r = -EINVAL;
-		if (len < sizeof(struct dtl_entry))
-			break;
-		if (addr && !vcpu->arch.vpa.next_gpa)
+		if (addr && (len < sizeof(struct dtl_entry) ||
+			     !vcpu->arch.vpa.next_gpa))
 			break;
 		len -= len % sizeof(struct dtl_entry);
 		r = set_vpa(vcpu, &vcpu->arch.dtl, addr, len);