netfilter: nft_rbtree: no need for spinlock from set destroy path

The sets are released from the rcu callback, after the rule is removed
from the chain list, which implies that nfnetlink cannot update the
rbtree and no packets are walking on the set anymore. Thus, we can get
rid of the spinlock in the set destroy path there.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Reviewied-by: Thomas Graf <tgraf@suug.ch>