rculfhash: make struct rcu_level size power of 2

By adding a small slowpath overhead (added synchronize_rcu call in the
last iteration of the resize), we can reduce the amount of wasted memory
for memory allocators that allocate power of two memory areas. This is
achieved by removing the call_rcu head from struct rcu_level.

[ Edit by Mathieu Desnoyers:
  - add comment about need to manually update the allocation size of
    fields are added to struct rcu_level.
  - create a more explanatory title and changelog. ]

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>