LU-9678 osd-zfs: register arc_prune_func() after site init

LU-9678 osd-zfs: register arc_prune_func() after site init

Register arc_prune_func() after site init, otherwise it may be
called while object cache is not initialized yet.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I822252da906f03899386fb0941cc11c1c3366fbf
Reviewed-on: https://review.whamcloud.com/27708
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>