The feature flags NETIF_F_TSO and NETIF_F_TSO6 independently enable
TSO for IPv4 and IPv6 respectively.  However, the test in
netdev_fix_features() and its predecessor functions was never updated
to check for NETIF_F_TSO6, possibly because it was originally proposed
that TSO for IPv6 would be dependent on both feature flags.

Now that these feature flags can be changed independently from
user-space and we depend on netdev_fix_features() to fix invalid
feature combinations, it's important to disable them both if
scatter-gather is disabled.  Also disable NETIF_F_TSO_ECN so
user-space sees all TSO features as disabled.
Signed-off-by: NBen Hutchings <bhutchings@solarflare.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>