netfilter: nf_flow_table_offload: check the status of dst_neigh

It is better to get the dst_neigh with neigh->lock and check the
nud_state is VALID. If there is not neigh previous, the lookup will
Create a non NUD_VALID with 00:00:00:00:00:00 mac.

Fixes: c29f74e0

 ("netfilter: nf_flow_table: hardware offload support")
Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>