In current design, legacy parser rte_devargs_parse() saved scratch
buffer to devargs.args while new parser rte_devargs_layers_parse() saved
to devargs.data. Code using devargs had to know the difference and
cleaned up memory accordingly - error prone.

This patch unifies data the dedicate scratch buffer, introduces
rte_devargs_free() function to wrap the memory memory clean up.

Signed-off-by: Xueming Li <redacted>
---

--- a/lib/librte_eal/include/rte_devargs.h
+++ b/lib/librte_eal/include/rte_devargs.h

@@ -60,16 +60,16 @@ struct rte_devargs {
 	/** Name of the device. */
 	char name[RTE_DEV_NAME_MAX_LEN];
 	RTE_STD_C11
-	union {
-	/** Arguments string as given by user or "" for no argument. */
-		char *args;
+	union { /**< driver-related part of device string. */
+		const char *args; /**< legacy name. */
 		const char *drv_str;
 	};
 	struct rte_bus *bus; /**< bus handle. */
 	struct rte_class *cls; /**< class handle. */
 	const char *bus_str; /**< bus-related part of device string. */
 	const char *cls_str; /**< class-related part of device string. */
-	const char *data; /**< Device string storage. */
+	char *data; /**< Scratch buffer. */
+	const char *src; /**< Arguments given by user. */