netfilter: nft_rbtree: introduce locking

There's no rbtree rcu version yet, so let's fall back on the spinlock
to protect the concurrent access of this structure both from user
(to update the set content) and kernel-space (in the packet path).

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

diff --cc net/netfilter/nft_rbtree.c
Simple merge