LU-4406 osd-zfs: Correct number of integers for zap key

LU-4406 osd-zfs: Correct number of integers for zap key

All zap_*_uint64 functions take a key size that is the number of
uint64s.  This corrects the osd_prepare_key to account for that, and
changes the name to make it more consistant with zap functions.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I8ee5ee6e955016fc4340025cede21aaf5bd034b7
Reviewed-on: http://review.whamcloud.com/8857
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>