LU-3489 osd-zfs: object iterator support

LU-3489 osd-zfs: object iterator support

a trivial object iterator for ZFS using dmu_object_next().
the iterator skips all objects with no LMA EA. It does
implement simple prefetching algoright. It also enables
few tests in sanity-lfsck to be run with ZFS.

Test-Parameters: mdtfilesystemtype=zfs \
    ostfilesystemtype=zfs mdsfilesystemtype=zfs \
    testlist=sanity-scrub,sanity-lfsck
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: Icf02342ac351a163e23575963d3366761c59b02b
Reviewed-on: http://review.whamcloud.com/7149
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>