netfilter: nft_flow_offload: missing netlink attribute policy

[ Upstream commit 14c415862c0630e01712a4eeaf6159a2b1b6d2a4 ]

The netlink attribute policy for NFTA_FLOW_TABLE_NAME is missing.

Fixes: a3c90f7a ("netfilter: nf_tables: flow offload expression")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>