If we forward a fragmented packet, we can have ip_summed
set to CHECKSUM_HW or similar.  This is fine for local
protocol processing, but once if we are forwarding this
packet we want to reset ip_summed to CHECKSUM_NONE.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>