osd: tiering: allow proxy read on background promotion

This is a correction of commit #22cd6f8. Fix the place to allow proxy
read.

Signed-off-by: Zhiqiang Wang <zhiqiang.wang@intel.com>
osd: tiering: avoid duplicate promotion on proxy read

Do not promote if it is already undergoing in maybe_handle_cache.

Signed-off-by: Zhiqiang Wang <zhiqiang.wang@intel.com>