decrement WBThrottle perfcounters in clear_object

We observed that the WBThrottle perfcounters are leaking upwards
at a rate of around 50-100 ios_dirtied per day. The counters are
currently not decremented in clear_object, so that's the likely
explanation. Decrement them like elsewhere in WBThrottle.

Signed-off-by: Dan van der Ster <daniel.vanderster@cern.ch>