vchoi_fork

There is performance issue when closing an object.  The slow down is due to the search of
the "tag_list" to find out the "corked" status of an object.
The fix:
(1)  Add a counter "num_objs_corked" in the cache structure to track the number
of "corked" objects.
(2) Skip the search of "tag_list" if the counter is zero i.e. no "corked" objects.