LU-4684 xattr: add list support for remote object

LU-4684 xattr: add list support for remote object

XATTR_LIST may be issued to a remote object in directory migration,
add this support for OSP and OUT.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I9681e149703de2837a04dc1448d1bd583659205d
Reviewed-on: https://review.whamcloud.com/31426
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>