dm cache: io destined for the cache device can now serve as tick bios

Previously only origin bios could trigger ticks, which meant if all
the io was destined for the cache no ticks were generated.  If no ticks
are generated then multiple hits, and movements in general, are
attributed to the same tick.

Only a stop gap fix, we need a better solution.

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