net: bonding, dummy, ifb, team: advertise NETIF_F_GSO_SOFTWARE

Virtual netdevs should use NETIF_F_GSO_SOFTWARE to forward GSO skbs
as-is and let the final drivers deal with them when supported.
Also remove NETIF_F_GSO_UDP_L4 from bonding and team drivers as it's
now included in the "software" list.

Suggested-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>