vsock: Add support for SIOCINQ ioctl

Add support for SIOCINQ ioctl, indicating the length of bytes unread in the
socket. The value is obtained from `vsock_stream_has_data()`.

Signed-off-by: Xuewei Niu <niuxuewei.nxw@antgroup.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Luigi Leonardi <leonardi@redhat.com>
Link: https://patch.msgid.link/20250708-siocinq-v6-2-3775f9a9e359@antgroup.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: justxuewei

net/vmw_vsock/af_vsock.c

@@ -1389,6 +1389,28 @@ static int vsock_do_ioctl(struct socket *sock, unsigned int cmd,
 	vsk = vsock_sk(sk);
 
 	switch (cmd) {
+	case SIOCINQ: {
+		ssize_t n_bytes;
+
+		if (!vsk->transport) {
+			ret = -EOPNOTSUPP;
+			break;
+		}
+
+		if (sock_type_connectible(sk->sk_type) &&
+		    sk->sk_state == TCP_LISTEN) {
+			ret = -EINVAL;
+			break;
+		}
+
+		n_bytes = vsock_stream_has_data(vsk);
+		if (n_bytes < 0) {
+			ret = n_bytes;
+			break;
+		}
+		ret = put_user(n_bytes, arg);
+		break;
+	}
 	case SIOCOUTQ: {
 		ssize_t n_bytes;