The rhashtable API has changed over time. Continuing to use it means
having to worry about maintaining different APIs in different kernel
generations.
We have a static pool of cursors so we don't need the flexibility of the
resizable rhashtable. We can roll a simple array of hlist heads to use
as a hash table.
And finally, these cursors will probably disappear eventually anyway.
Let's not invest too much in them.
Signed-off-by: Zach Brown <zab@versity.com>