MINOR: samples: rename a struct from sample_storage to sample_data

This a first step of sample reorganization.
diff --git a/include/types/vars.h b/include/types/vars.h
index c387a77..14cb33d 100644
--- a/include/types/vars.h
+++ b/include/types/vars.h
@@ -27,7 +27,7 @@
 struct var {
 	struct list l; /* Used for chaining vars. */
 	const char *name; /* Contains the variable name. */
-	struct sample_storage data; /* data storage. */
+	struct sample_data data; /* data storage. */
 };
 
 #endif