osd/ReplicatedPG: add slop to agent mode selection

We want to avoid a situation where the agent clicks on and off when the
system hovers around a utilization threshold.  Particularly for trim,
the system can expend a lot of energy doing a minimal amount of work when
the effort level is low.  To avoid this, enable when we are some amount
above the threshold, and do not turn off until we are the same amount below
the target.

Signed-off-by: Sage Weil <sage@inktank.com>
osd/ReplicatedPG: fix evict mode selection for large target

If the target is > 1.0 for some reason (bad configuration, or high slop
value), and we are not yet full, we should be in IDLE mode--not SOME.

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