Add a tunable to disable BRT support.

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