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/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