netfilter: nf_flow_table: attach dst to skbs

Some drivers, such as vxlan and wireguard, use the skb's dst in order to
determine things like PMTU. They therefore loose functionality when flow
offloading is enabled. So, we ensure the skb has it before xmit'ing it
in the offloading path.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>