netfilter: nftables: rename set element data activation/deactivation functions

Rename:

- nft_set_elem_activate() to nft_set_elem_data_activate().
- nft_set_elem_deactivate() to nft_set_elem_data_deactivate().

To prepare for updates in the set element infrastructure to add support
for the special catch-all element.

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