Destroy the rw-lock object before freeing the memory it is occupying.
If we do not do this, then the mutex that is contained in the rw-lock
object btr_search_latch_temp->mutex gets "freed" and subsequently
mutex_free() from sync_close() hits a mutex whose memory has been
freed and crashes.

Approved by:	Heikki (via IRC)
Discussed with:	Calvin