FreeBSD: fix unpropagated error

When performing I/O on FreeBSD using a file based vdev ensure all
errors encountered when reading/writing are propagated through the
zio pipeline.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12904