osd-zfs should mark some transactions netfree. this means those
transactions are expected to release space (rather than consume)
and for this kind of transaction half of reserved space is available.

Change-Id: Ia5ca247843b296319376c4ac69efad68b557df9f
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: https://review.whamcloud.com/31444


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>