netfilter: nft_set_rbtree: use seqcount to avoid lock in most cases

switch to lockless lockup. write side now also increments sequence
counter.  On lookup, sample counter value and only take the lock
if we did not find a match and the counter has changed.

This avoids need to write to private area in normal (lookup) cases.

In case we detect a writer (seqretry is true) we fall back to taking
the readlock.

The readlock is also used during dumps to ensure we get a consistent
tree walk.

Similar technique (rbtree+seqlock) was used by David Howells in rxrpc.

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