tunables: ensure tunable and variable have same define gate

If a variable is only available in the kernel, then the tunable should
also only be available there.

This matters very little so long as we don't have userspace tunables,
but its still good hygeine.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Pavel Snajdr <snajpa@snajpa.net>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17377