netfilter: nft_set_rbtree: rename gc deactivate+erase function

Next patch adds a cllaer that doesn't hold the priv->write lock and
will need a similar function.

Rename the existing function to make it clear that it can only
be used for opportunistic gc during insertion.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>