net: lib: zperf: fix internal UDP header definition

Fix the zperf UDP datagram header definition, as iperf introduced a
backwards incompatible format change in version 2.0.10 (August 2017).
```
struct UDP_datagram {
// used to reference the 4 byte ID number we place in UDP datagrams
// Support 64 bit seqno on machines that support them
    uint32_t id;
    uint32_t tv_sec;
    uint32_t tv_usec;
    uint32_t id2;
};
```

Update the header to the new format, with a Kconfig option to fall back
to the previous header definition.

The response decoding was testd with a nRF7002 client and
`iperf-2.2.1-win64.exe` server, with the output statistics struct now
containing the same information as reported on the PC server.

Signed-off-by: Jordan Yates <jordan@embeint.com>

Author: JordanYates

subsys/net/lib/zperf/Kconfig

@@ -13,6 +13,14 @@ menuconfig NET_ZPERF
 
 if NET_ZPERF
 
+config NET_ZPERF_LEGACY_HEADER_COMPAT
+	bool "Legacy iperf UDP header format"
+	help
+	  iperf 2.0.10 (2017) updated the UDP header format in a
+	  backwards incompatible manner that cannot be automatically
+	  detected. This option reverts the header format for use with
+	  iperf version 2.0.9 and earlier.
+
 config ZPERF_WORK_Q_THREAD_PRIORITY
 	int "zperf work queue thread priority"
 	default NUM_PREEMPT_PRIORITIES

subsys/net/lib/zperf/zperf_internal.h

@@ -54,9 +54,12 @@
 #define ZPERF_VERSION "1.1"
 
 struct zperf_udp_datagram {
-	int32_t id;
+	uint32_t id;
 	uint32_t tv_sec;
 	uint32_t tv_usec;
+#ifndef CONFIG_NET_ZPERF_LEGACY_HEADER_COMPAT
+	uint32_t id2;
+#endif
 } __packed;
 
 BUILD_ASSERT(sizeof(struct zperf_udp_datagram) <= PACKET_SIZE_MAX, "Invalid PACKET_SIZE_MAX");