ethdev: add mbuf RSS update as an offload

Add new Rx offload flag `DEV_RX_OFFLOAD_RSS_HASH` which can be used to
enable/disable PMDs write to `rte_mbuf::hash::rss`.
PMDs notify the validity of `rte_mbuf::hash:rss` to the application
by enabling `PKT_RX_RSS_HASH ` flag in `rte_mbuf::ol_flags`.

Also update testpmd rx_offload command to include RSS_HASH

Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>