loopback: Enable TSO

This patch enables TSO since the loopback device is naturally
capable of handling packets of any size.  This also means that
we won't enable GSO on lo which is good until GSO is fixed to
preserve netfilter state as netfilter treats loopback packets
in a special way.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>