osd: tiering: calculate object age during eviction when there is no hit
set

Signed-off-by: Zhiqiang Wang <zhiqiang.wang@intel.com>
Merge pull request #2826 from wonzhq/evict-atime-nohitset

osd: tiering: calculate object age during eviction when there is no hit set

Reviewed-by: Sage Weil <sage@redhat.com>