netfilter: nf_fwd_netdev: clear timestamp in forwarding path

Similar to 7980d2ea ("ipvs: clear skb->tstamp in forwarding path").
fq qdisc requires tstamp to be cleared in forwarding path.

Fixes: 8203e2d8 ("net: clear skb->tstamp in forwarding paths")
Fixes: fb420d5d ("tcp/fq: move back to CLOCK_MONOTONIC")
Fixes: 80b14dee

 ("net: Add a new socket option for a future transmit time.")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>