netfilter: nf_tables: set->ops->insert returns opaque set element in case of EEXIST

Return struct nft_elem_priv instead of struct nft_set_ext for
consistency with ("netfilter: nf_tables: expose opaque set element as
struct nft_elem_priv") and to prepare the introduction of element
timeout updates from control path.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>