FileJournal: Add ssd discard for journal which using ssd disk as journal.

Journal is like a ring buffer. After data wrote to media disk, journal
can overwrite. But for those data, ssd dont't know it's nouse and can
remove. So add discard to tell ssd 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>
FileJournal: add journal_discard to control ssd whether support discard

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>