dm cache: track all IO to the cache rather than just the origin device's IO

IO tracking used to throttle writebacks when the origin device is busy.

Even if all the IO is going to the fast device, writebacks can
significantly degrade performance.  So track all IO to gauge whether the
cache is busy or not.

Otherwise, synthetic IO tests (e.g. fio) that might send all IO to the
fast device wouldn't cause writebacks to get throttled.

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