Add a tunable to disable BRT support.

Copy the disable parameter that FreeBSD implemented, and extend it to
work on Linux as well, until we're sure this is stable.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #15529

module/os/linux/zfs/zfs_vnops_os.c

@@ -4249,4 +4249,8 @@ EXPORT_SYMBOL(zfs_map);
 module_param(zfs_delete_blocks, ulong, 0644);
 MODULE_PARM_DESC(zfs_delete_blocks, "Delete files larger than N blocks async");
 
+/* CSTYLED */
+module_param(zfs_bclone_enabled, uint, 0644);
+MODULE_PARM_DESC(zfs_bclone_enabled, "Enable block cloning");
+
 #endif