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: Christoph Hellwig <hch@lst.de> Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>