graph: implement fastpath routines

Adding implementation for rte_graph_walk() API. This will perform a walk
on the circular buffer and call the process function of each node
and collect the stats if stats collection is enabled.

Signed-off-by: Jerin Jacob <jerinj@marvell.com>
Signed-off-by: Kiran Kumar K <kirankumark@marvell.com>
Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
Signed-off-by: Nithin Dabilpuram <ndabilpuram@marvell.com>