netfilter: nf_flow_table: clear skb tstamp before xmit

If 'fq' qdisc is used and a program has requested timestamps,
skb->tstamp needs to be cleared, else fq will treat these as
'transmit time'.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>