Condition variable reference counts

Reference count every entry and exit from the condition variable
functions: cv_wait(), cv_wait_timeout(), cv_signal(), cv_broadcast().

This allows us to safely block in cv_destroy() until all consumers
have been scheduled and are no longer accessing the condition
variable memory.

In addition poison the magic value at the start of cv_destroy() to
ensure there are never any new callers after cv_destroy() is called.
The consumer is responsible for ensuring this never occurs.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

include/sys/condvar.h

@@ -35,12 +35,13 @@
  * calling any of the wait/signal funs, and passed into the wait funs.
  */
 #define CV_MAGIC			0x346545f4
-#define CV_POISON			0x95
+#define CV_DESTROY			0x346545f5
 
 typedef struct {
 	int cv_magic;
 	wait_queue_head_t cv_event;
 	wait_queue_head_t cv_destroy;
+	atomic_t cv_refs;
 	atomic_t cv_waiters;
 	kmutex_t *cv_mutex;
 } kcondvar_t;