FreeBSD: Use a statement expression to implement SET_ERROR() (#16284)

This way we can avoid making assumptions about the SDT probe
implementation. No functional change intended.

Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>