zio-sysc.c: fix bug from patch 896e0664 and improve comments

The patch 896e0664 misses to assign the zattr for the zio
extended attributes. This patch also improve comments to avoid
similar errors in the future.

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
Acked-by: Alessandro Rubini <rubinig@gnudd.com>