loopback: make use of NETIF_F_GSO_SOFTWARE

NETIF_F_GSO_SOFTWARE was defined to list all GSO software types, so lets
make use of it in loopback code. Note that veth/vxlan/others already
uses it.

Within this patch series, this patch causes lo to pick up SCTP GSO feature
automatically (as it's added to NETIF_F_GSO_SOFTWARE) and thus avoiding
segmentation if possible.

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Tested-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>