i40e: allow toggling loopback mode via ndo_set_features callback

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

This sets the MAC Tx->Rx loopback.

This feature is used for the xsk selftests, and might have other uses
too.

Signed-off-by: Tirthendu Sarkar <tirthendu.sarkar@intel.com>
Reviewed-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Tested-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Link: https://lore.kernel.org/r/20221209185553.2520088-1-anthony.l.nguyen@intel.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>