netfilter: nft_rbtree: check for next generation when deactivating elements

netfilter: nft_rbtree: check for next generation when deactivating elements

set->ops->deactivate() is invoked from nft_del_setelem() that happens
from the transaction path, so we have to check if the object is active
in the next generation, not the current.

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