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

man/man5/zfs-module-parameters.5

@@ -1,6 +1,6 @@
 '\" te
 .\" Copyright (c) 2013 by Turbo Fredriksson <turbo@bayour.com>. All rights reserved.
-.\" Copyright (c) 2019, 2020 by Delphix. All rights reserved.
+.\" Copyright (c) 2019, 2021 by Delphix. All rights reserved.
 .\" Copyright (c) 2019 Datto Inc.
 .\" The contents of this file are subject to the terms of the Common Development
 .\" and Distribution License (the "License").  You may not use this file except
@@ -691,6 +691,29 @@ will occur.
 Default value: \fB600000\fR (ten minutes).
 .RE
 
+.sp
+.ne 2
+.na
+\fBreference_history\fR (int)
+.ad
+.RS 12n
+Maximum reference holders being tracked when reference_tracking_enable is
+active.
+.sp
+Default value: \fB3\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBreference_tracking_enable\fR (int)
+.ad
+.RS 12n
+Track reference holders to refcount_t objects (debug builds only).
+.sp
+Use \fB1\fR for yes and \fB0\fR for no (default).
+.RE
+
 .sp
 .ne 2
 .na