LU-8928 osd: convert osd-zfs to reference dnode, not db

LU-8928 osd: convert osd-zfs to reference dnode, not db

this will be used later with methods like zap_add_by_dnode()
and similar, which are significantly faster as they don't
need to lookup dnode by dnode#.

Change-Id: Idc5341e9a472bbf0e5088b1bee784e4ddb6d635b
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: https://review.whamcloud.com/24293
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>