Reduce the default UDP send size to 1452 to avoid IP fragmentation

with default parameters.

A UDP payload of 1452, plus an 8-byte UDP header, plus a 40-byte IPv6
header, results in a 1500 byte IP packet.  The IPv4 header is smaller
at 20 bytes.

src/iperf_api.h

@@ -39,7 +39,7 @@ struct iperf_stream;
 #define Ptcp SOCK_STREAM
 #define Pudp SOCK_DGRAM
 #define Psctp 12
-#define DEFAULT_UDP_BLKSIZE 8192
+#define DEFAULT_UDP_BLKSIZE 1452
 #define DEFAULT_TCP_BLKSIZE (128 * 1024)  /* default read/write block size */
 #define DEFAULT_SCTP_BLKSIZE (64 * 1024)