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>