LU-1864 osd-zfs: Use the correct lu_site in arc_prune_func()

LU-1864 osd-zfs: Use the correct lu_site in arc_prune_func()

arc_prune_func() purges osd_device::od_site.  While this is correct on
orion and master, b2_3 does not initialize or use osd_device::od_site
at all.  This patch changes arc_prune_func() to use the correct
lu_site from osd_device::...::ld_site, which points to
ofd_device::ofd_site.

Change-Id: I2344886fd9bd5f6166907489c772f93a717ed5ad
Signed-off-by: Li Wei <liwei@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/3988
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>