netfilter: nf_tables: validate NFTA_SET_ELEM_OBJREF based on NFT_SET_OBJECT flag

If the NFTA_SET_ELEM_OBJREF netlink attribute is present and
NFT_SET_OBJECT flag is set on, report EINVAL.

Move existing sanity check earlier to validate that NFT_SET_OBJECT
requires NFTA_SET_ELEM_OBJREF.

Fixes: 8aeff920

 ("netfilter: nf_tables: add stateful object reference to set elements")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>