Cache tiering: adjust avg_size calculation in agent_choose_mode

Two things to correct in agent_choose_mode:
1) When info.stats.stats.sum.num_objects is greater than 0, but num_user_objects is not greater than 0, there is no need to calculate dirty_micro and full_micro.
2) Usually, the size of a hit set object is relative small comparing to a data object. So it would be better to exclude the hit set objects when calculating avg_size. When the number of hit sets are not that small, it makes differences.
Signed-off-by: Zhiqiang Wang <wonzhq@hotmail.com>