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: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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