osd: cache tiering: fix the atime logic of the eviction

Reported-by: Xinze Chi <xmdxcxz@gmail.com>
Signed-off-by: Zhiqiang Wang <zhiqiang.wang@intel.com>
Merge pull request #2815 from wonzhq/evict-atime

osd: cache tiering: fix the atime logic of the eviction

Reviewed-by: Sage Weil <sage@redhat.com>