netfilter: nf_tables: tighten netlink attribute requirements for catch-all elements

[ Upstream commit d4eb7e39 ]

If NFT_SET_ELEM_CATCHALL is set on, then userspace provides no set element
key. Otherwise, bail out with -EINVAL.

Fixes: aaa31047 ("netfilter: nftables: add catch-all set element support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>