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

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.  Handle '..' like '.'
when retrieving ZAP values.

Test-Parameters: mdtfilesystemtype=zfs mdsfilesystemtype=zfs ostfilesystemtype=zfs testlist=conf-sanity,conf-sanity,conf-sanity,conf-sanity
Signed-off-by: Nathaniel Clark <nathaniel.l.clark@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>
Change-Id: I916573c70c8828bed6905b5eda9344b4a49b7f11
Reviewed-on: http://review.whamcloud.com/12904
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>