MINOR: samples: rename union from "data" to "u"

The union name "data" is a little bit heavy while we read the source
code because we can read "data.data.sint". The rename from "data" to "u"
makes the read easiest like "data.u.sint".
diff --git a/include/types/sample.h b/include/types/sample.h
index 6d30943..1ec9eed 100644
--- a/include/types/sample.h
+++ b/include/types/sample.h
@@ -253,7 +253,7 @@
 /* Used to store sample constant */
 struct sample_data {
 	int type;                 /* SMP_T_* */
-	union sample_value data;  /* sample data */
+	union sample_value u;     /* sample data */
 };
 
 /* a sample is a typed data extracted from a stream. It has a type, contents,