netfilter: nftables: report EOPNOTSUPP on unsupported flowtable flags

[ Upstream commit 7e6136f1 ]

Error was not set accordingly.

Fixes: 8bb69f3b ("netfilter: nf_tables: add flowtable offload control plane")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>