LU-7863 osd-zfs: dmu_prefetch change in ZFS master

LU-7863 osd-zfs: dmu_prefetch change in ZFS master

In ZFS master (post zfs-0.6.5-release) dmu_prefetch now has 6
arguments instead of 4.  This accounts for the new style dmu_prefetch,
while keeping backward compatibility (back to 0.6.2 which initially
exported dmu_prefetch).

Convert old behavior of SYNC_READ for prefetch to ASYNC_READ as a
better prefetch behavior.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Ia06e5605b06ef1f51817418183e479d591231e5a
Reviewed-on: http://review.whamcloud.com/19293
Tested-by: Jenkins
Reviewed-by: Christopher J. Morrone <morrone2@llnl.gov>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>