FileJournal: Add ssd discard for journal which use ssd block as journal

Journal is like a ring buffer. After data wrote to media disk, journal
can overwrite. But for those data, ssd don't know it's no use and
can remov. So add discard to tell sdd to remove those data.
This maybe not increase the performance. But it can increase the
lifetime of ssd.
Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>
maybe do_discard or _discard if it ends up being bool discard above