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