Reference_tracking_enable should be a module param

To make use of zfs_refcount_held tunable it should be a module 
parameter in open-zfs.  Also, since the macros will auto-generate OS 
specific tunables, removed the existing zfs_refcount_held reference 
in module/os/freebsd/zfs/sysctl_os.c.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Don Brady <don.brady@delphix.com>
Closes #11753

module/zfs/refcount.c

@@ -20,7 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2012, 2015 by Delphix. All rights reserved.
+ * Copyright (c) 2012, 2021 by Delphix. All rights reserved.
  */
 
 #include <sys/zfs_context.h>
@@ -324,4 +324,12 @@ zfs_refcount_not_held(zfs_refcount_t *rc, const void *holder)
 	mutex_exit(&rc->rc_mtx);
 	return (B_TRUE);
 }
+
+/* BEGIN CSTYLED */
+ZFS_MODULE_PARAM(zfs, ,reference_tracking_enable, INT, ZMOD_RW,
+	"Track reference holders to refcount_t objects");
+
+ZFS_MODULE_PARAM(zfs, ,reference_history, INT, ZMOD_RW,
+	"Maximum reference holders being tracked");
+/* END CSTYLED */
 #endif	/* ZFS_DEBUG */