compare hash value before call compare_fct()

[ Edit by Mathieu Desnoyers:

  This is an optimisation that checks if the reverse hash value is
  equal before calling the comparison function. Given comparing a
  reverse hash is much faster than comparison, this accelarates the
  lookups and add. ]

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