selftests: mptcp: refine the 'iovlen' handling logic

In the original version, the handling logic for the 'iovlen' appeared
somewhat redundant. Therefore, 'iovlen++' is used to simpify this
portion of the code and improve its readaility.

Co-developed-by: Geliang Tang <[email protected]>
Signed-off-by: Geliang Tang <[email protected]>
Signed-off-by: Gang Yan <[email protected]>

Author: Dwyane-Yan

tools/testing/selftests/net/mptcp/mptcp_diag.c

@@ -91,10 +91,9 @@ static void send_query(int fd, struct inet_diag_req_v2 *r)
 		.iov_base = &req,
 		.iov_len = sizeof(req)
 	};
-	iov[iovlen] = (struct iovec){ &rta_proto, sizeof(rta_proto)};
-	iov[iovlen + 1] = (struct iovec){ &proto, sizeof(proto)};
+	iov[iovlen++] = (struct iovec){ &rta_proto, sizeof(rta_proto)};
+	iov[iovlen++] = (struct iovec){ &proto, sizeof(proto)};
 	req.nlh.nlmsg_len += RTA_LENGTH(sizeof(proto));
-	iovlen += 2;
 	struct msghdr msg = {
 		.msg_name = &nladdr,
 		.msg_namelen = sizeof(nladdr),