Add support for NETIF_F_LOOPBACK. This feature can be set via:
$ ethtool -K eth0 loopback <on|off>

Feature can be useful for local data path tests.
Acked-by: NJakub Kicinski <kuba@kernel.org>
Acked-by: NJohn Fastabend <john.fastabend@gmail.com>
Signed-off-by: NMaciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: NGeorge Kuruvinakunnel <george.kuruvinakunnel@intel.com>
Signed-off-by: NTony Nguyen <anthony.l.nguyen@intel.com>