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

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>