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>
Do You think it is a better idea to read "/sys/block/<devname>/queue/discard_granularity" and check if it is non-zero than depending on ioctl/errno? I am not sure what is the side effect of sending this operation (range of 0,0) on a device which does support block discards.