LU-10174 osd-zfs: Fix build against ZFS 0.6.5.x

LU-10174 osd-zfs: Fix build against ZFS 0.6.5.x

Ensure all values used in https://review.whamcloud.com/29242 are
actually used conditionally.  This allows us to build against
ZFS 0.6.5.7 again.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Ide560e21f7b7fedc0c107b1943602e5af99b478c
Reviewed-on: https://review.whamcloud.com/29866
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>