LU-3573 osd-zfs: Only advance zap cursor as needed

Only advance the zap cursor when ozi_pos is not advanced, otherwise
occasionally the a file could get "lost" because the zap_cursor would
advance over it before the retrieve happened.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Iad560e2ffb4cfe2c74a1cf9197be7c2537538822
Reviewed-on: http://review.whamcloud.com/12582
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Isaac Huang <he.huang@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>