Use workqueue in rculfhash

The RCU lock-free hash table currently requires that the destroy
function should not be called from within RCU read-side critical
sections. This is caused by the lazy resize, which uses the call_rcu
worker thread, even though all it really needs is a workqueue/worker
thread scheme.

Use the new internal workqueue API instead of call_rcu in rculfhash to
overcome this limitation.

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