dm cache policy mq: fix promotions to occur as expected

Micro benchmarks that repeatedly issued IO to a single block were
failing to cause a promotion from the origin device to the cache.  Fix
this by not updating the stats during map() if -EWOULDBLOCK will be
returned.

The mq policy will only update stats, consider migration, etc, once per
tick period (a unit of time established between dm-cache core and the
policies).

When the IO thread calls the policy's map method, if it would like to
migrate the associated block it returns -EWOULDBLOCK, the IO then gets
handed over to a worker thread which handles the migration.  The worker
thread calls map again, to check the migration is still needed (avoids a
race among other things).  *BUT*, before this fix, if we were still in
the same tick period the stats were already updated by the previous map
call -- so the migration would no longer be requested.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

diff --cc drivers/md/dm-cache-policy-mq.c
Simple merge