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

module/os/freebsd/zfs/vdev_file.c

@@ -234,6 +234,7 @@ vdev_file_io_strategy(void *arg)
 		err = zfs_file_pwrite(vf->vf_file, buf, size, off, &resid);
 		abd_return_buf(zio->io_abd, buf, size);
 	}
+	zio->io_error = err;
 	if (resid != 0 && zio->io_error == 0)
 		zio->io_error = ENOSPC;