add rculfhash-mm-mmap.c memory management

[ Edit by Mathieu Desnoyers:
  - change "buckets" for "mmap" to better show the mapping between the
    union member and the mm plugin.
  - 80 col coding style fixups. ]

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