osd: trigger the cache agent after a promotion

osd: trigger the cache agent after a promotion

When a proxy read happens, the object promotion is done in parallel. The
agent_choose_mode function must be called to reconsider the situation
to protect against the following scenario:

  * proxy read
  * agent_choose_mode finds no object exists and the agent
    goes idle
  * object promotion happens
  * the agent does not reconsider and eviction does not happen
    although it should

http://tracker.ceph.com/issues/12673 Fixes: #12673

Signed-off-by: Loic Dachary <ldachary@redhat.com>