netfilter: nf_flow_table_ip: reset mac header before vlan push

netfilter: nf_flow_table_ip: reset mac header before vlan push

With double vlan tagged packets in the fastpath, getting the error:

skb_vlan_push got skb with skb->data not at mac header (offset 18)

Call skb_reset_mac_header() before calling skb_vlan_push().

Fixes: c653d5a78f34 ("netfilter: flowtable: inline vlan encapsulation in xmit path")
Signed-off-by: Eric Woudstra <ericwouds@gmail.com>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>