ethdev: support encapsulation level for RSS offload

ethdev: support encapsulation level for RSS offload

This patch reserves 2 bits as input selection to select inner and outer
encapsulation level for RSS computation. It is combined with existing
ETH_RSS_* to choose inner or outer layers.
This functionality already exists in rte_flow through level parameter in
RSS action configuration rte_flow_action_rss.

Signed-off-by: Kiran Kumar K <kirankumark@marvell.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>