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

@@ -4242,4 +4242,8 @@ 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

module/os/linux/zfs/zpl_file_range.c

@@ -31,6 +31,8 @@
 #include <sys/zfs_vnops.h>
 #include <sys/zfeature.h>
 
+int zfs_bclone_enabled = 1;
+
 /*
  * Clone part of a file via block cloning.
  *
@@ -50,6 +52,9 @@ __zpl_clone_file_range(struct file *src_file, loff_t src_off,
 	fstrans_cookie_t cookie;
 	int err;
 
+	if (!zfs_bclone_enabled)
+		return (-EOPNOTSUPP);
+
 	if (!spa_feature_is_enabled(
 	    dmu_objset_spa(ITOZSB(dst_i)->z_os), SPA_FEATURE_BLOCK_CLONING))
 		return (-EOPNOTSUPP);