netfilter: nft_compat: ebtables emulation must reject non-bridge targets

netfilter: nft_compat: ebtables emulation must reject non-bridge targets

xtables targets return netfilter verdicts: NF_ACCEPT, NF_DROP, and so
on.  ebtables targets return incompatible verdicts: EBT_ACCEPT,
EBT_DROP, ...   We cannot allow fallback to NFPROTO_UNSPEC.

ebtables doesn't permit this since
11ff7288beb2 ("netfilter: ebtables: reject non-bridge targets")
but that commit missed the nft_compat layer.

Reported-by: Ren Wei <n05ec@lzu.edu.cn>
Reported-by: Wyatt Feng <bronzed_45_vested@icloud.com>
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Zhengchuan Liang <zcliangcn@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Fixes: 0ca743a55991 ("netfilter: nf_tables: add compatibility layer for x_tables")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>