rculfhash updates

- Add and remove now completely lock-free (no busy-wait loop).
- Add resize_initiated flag to skip multiple call_rcu calls.
- Identical reverse hashes now only count as one chain entry in the
  chain length accounting. We cannot split identical hashes by growing
  the hash table anyway, so it was leading to large tables uselessly
  when we had a lot of similar hashes.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>