netfilter: nft_flow_offload: zero device address for non-ether case

netfilter: nft_flow_offload: zero device address for non-ether case

LLM points out that the skip causes unitialised stack array to
propagate down into dev_fill_forward_path().  Its not clear to me that
there is a guarantee that a later ctx.dev->netdev_ops->ndo_fill_forward_path()
would always fix this up.

Cc: Felix Fietkau <nbd@nbd.name>
Fixes: 45ca3e61999e ("netfilter: nft_flow_offload: skip dst neigh lookup for ppp devices")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>