ethdev: add flow action to swap MAC addresses

ethdev: add flow action to swap MAC addresses

This action is useful for offloading loopback mode, where the hardware
will swap source and destination MAC addresses in the outermost Ethernet
header before looping back the packet. This action can be used in
conjunction with other rewrite actions to achieve MAC layer transparent
NAT where the MAC addresses are swapped before either the source or
destination MAC address is rewritten and NAT is performed.

Must be used with a valid RTE_FLOW_ITEM_TYPE_ETH flow pattern item.
Otherwise, RTE_FLOW_ERROR_TYPE_ACTION error should be returned by the
PMDs.

Original work by Shagun Agrawal

Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>