Cache tiering: use local_mtime to calculate the age of objects during
evicting

Commit 13b9dc70 introduces local_mtime to fix an issue when doing
flushing. Should also use this to calculate the object age when doing
evicting.

Signed-off-by: Zhiqiang Wang <wonzhq@hotmail.com>
Merge pull request #2352 from wonzhq/local-mtime

Cache tiering: use local_mtime to calculate the age of objects during evicting

Reviewed-by: Sage Weil <sage@redhat.com>