LU-2803 osd: osd-zfs to handle echo sequence (2) properly

LU-2803 osd: osd-zfs to handle echo sequence (2) properly

use visible OI (/O directory) to map object from this sequence
into dnodes. this let obdecho to work with zfs.

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: I63e50789df502d11863c69658c9524fbb3cd9f22
Reviewed-on: http://review.whamcloud.com/5414
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>