netfilter: nft_compat: reject unused compat flag

[ Upstream commit 292781c3c5485ce33bd22b2ef1b2bed709b4d672 ]

Flag (1 << 0) is ignored is set, never used, reject it it with EINVAL
instead.

Fixes: 0ca743a5 ("netfilter: nf_tables: add compatibility layer for x_tables")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>