LU-2990 osd-zfs: locate cursor to specified pos via load API

LU-2990 osd-zfs: locate cursor to specified pos via load API

There was defect for zfs-based backend when traverse directory:
the dt_iteration API ::load() did not locate the cursor to the
specified position, then caused non-first directory readpage
RPC obtained repeated entries, and then caused readdir() loop
or unexpected dir hash collision.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I9c159f0f80677590869246dd9f30f0dfb9cc2fbc
Reviewed-on: http://review.whamcloud.com/5894
Tested-by: Hudson
Reviewed-by: Li Wei <wei.g.li@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>