devargs: unify scratch buffer storage

devargs: unify scratch buffer storage

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 scratch buffer to data field, introduces
rte_devargs_reset() function to wrap the memory clean up logic.

Signed-off-by: Xueming Li <xuemingl@nvidia.com>
Acked-by: Ray Kinsella <mdr@ashroe.eu>
Reviewed-by: Gaetan Rivet <grive@u256.net>