Currently gfs2 issues barrier unconditionally.  There are various reasons
to disable them, be that just for testing or for stupid devices flushing
large battert backed caches.  Add a nobarrier option that matches xfs and
btrfs for this.  Also add a symmetric barrier option to turn it back on
at remount time.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NSteven Whitehouse <swhiteho@redhat.com>