osd: agent can spin if in low flush mode and no evict work
if there isn't evict work, we will only start the low flush max ops, but won't go to sleep if we're below the high limit.
osd: consider high/low mode when putting agent to sleep
If we are in low flush mode, we may only get up to max_low_ops in flight,
in which case we may never go to sleep here.
Fix it by using the max_low_ops threshold when appropriate.
Note that agent_work() might start up more than this many ops (if there
are lots of evicts to do) currently, but I think it is fine if evicts go
a bit slower if we are in low mode. (Really, the high/low shouldn't be
tied to flushing specifically.)