osd: tiering: proxy instead of redirect read in writeback mode when the
cache pool is full

To preserve read op order

Signed-off-by: Zhiqiang Wang <zhiqiang.wang@intel.com>
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>
Merge pull request #3012 from wonzhq/proxy-read-cache-full

osd: tiering: proxy instead of redirect read in writeback mode when the cache pool is full