netfilter: nft_tunnel: restrict it to netdev family

[ Upstream commit 01e4092d ]

Only allow to use this expression from NFPROTO_NETDEV family.

Fixes: af308b94

 ("netfilter: nf_tables: add tunnel support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>