rename min_alloc_size/min_alloc_order

[ Mathieu Desnoyers:

  This clarifies that the amount of buckets allocated can differ from
  the number of buckets chained together, especially for small hash
  tables, where we don't free under min_nr_alloc_buckets. ]

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