osd/ReplicatedPG: check agent_mode if agent is enabled but hit_sets aren't

osd/ReplicatedPG: check agent_mode if agent is enabled but hit_sets aren't

It is probably not a good idea to try to run the tiering agent without a
hit_set to inform its actions, but it is technically possible.  For
example, one could simply blindly evict when we reach the full point.
However, this doesn't work because the agent mode is guarded by a hit_set
check, even though agent_setup() is not.  Fix that.

Signed-off-by: Sage Weil <sage@redhat.com>