dm-cache's smq policy tries hard to do it's work during the idle periods
when there is no IO.  But if there are no idle periods (eg, a long fio
run) we still need to allow some demotions and promotions to occur.

To achieve this, pass @idle=true to queue_promotion()'s
free_target_met() call so that free_target_met() doesn't short-circuit
the possibility of demotion simply because it isn't an idle period.

Fixes: b29d4986 ("dm cache: significant rework to leverage dm-bio-prison-v2")
Reported-by: John Harrigan <jharriga@redhat.com>
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>