Revert "net: ip, ipv6: handle gso skbs in forwarding path"

This reverts commit caa5344994778a2b4725b2d75c74430f76925e4a, which
was commit fe6cc55f3a9a053482a76f5a6b2257cee51b4663 upstream.  In 3.2,
the transport header length is not calculated in the forwarding path,
so skb_gso_network_seglen() returns an incorrect result.  We also have
problems due to the local_df flag not being set correctly.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>