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

tests/zfs-tests/include/libtest.shlib

@@ -3334,6 +3334,21 @@ function set_tunable_impl
 	esac
 }
 
+function save_tunable
+{
+	[[ ! -d $TEST_BASE_DIR ]] && return 1
+	[[ -e $TEST_BASE_DIR/tunable-$1 ]] && return 2
+	echo "$(get_tunable """$1""")" > "$TEST_BASE_DIR"/tunable-"$1"
+}
+
+function restore_tunable
+{
+	[[ ! -e $TEST_BASE_DIR/tunable-$1 ]] && return 1
+	val="$(cat $TEST_BASE_DIR/tunable-"""$1""")"
+	set_tunable64 "$1" "$val"
+	rm $TEST_BASE_DIR/tunable-$1
+}
+
 #
 # Get a global system tunable
 #

tests/zfs-tests/include/tunables.cfg

@@ -93,6 +93,7 @@ VOL_INHIBIT_DEV			UNSUPPORTED			zvol_inhibit_dev
 VOL_MODE			vol.mode			zvol_volmode
 VOL_RECURSIVE			vol.recursive			UNSUPPORTED
 VOL_USE_BLK_MQ			UNSUPPORTED			zvol_use_blk_mq
+BCLONE_ENABLED			zfs_bclone_enabled		zfs_bclone_enabled
 XATTR_COMPAT			xattr_compat			zfs_xattr_compat
 ZEVENT_LEN_MAX			zevent.len_max			zfs_zevent_len_max
 ZEVENT_RETAIN_MAX		zevent.retain_max		zfs_zevent_retain_max

tests/zfs-tests/tests/functional/block_cloning/cleanup.ksh

@@ -31,4 +31,8 @@ verify_runnable "global"
 
 default_cleanup_noexit
 
+if tunable_exists BCLONE_ENABLED ; then
+	log_must restore_tunable BCLONE_ENABLED
+fi
+
 log_pass

tests/zfs-tests/tests/functional/block_cloning/setup.ksh

@@ -33,4 +33,9 @@ fi
 
 verify_runnable "global"
 
+if tunable_exists BCLONE_ENABLED ; then
+    log_must save_tunable BCLONE_ENABLED
+    log_must set_tunable32 BCLONE_ENABLED 1
+fi
+
 log_pass