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

This reverts commit f1e62c8d484a57cefb5ecbfcf8d1887b6f4fb220.

Right idea, but it turns out to be a pretty high-impact change.
Need to rethink this, maybe with a more intelligent implementation
that checks the interface (or path?) MTU.

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 1452
+#define DEFAULT_UDP_BLKSIZE 8192
 #define DEFAULT_TCP_BLKSIZE (128 * 1024)  /* default read/write block size */
 #define DEFAULT_SCTP_BLKSIZE (64 * 1024)