[GFS2] Fix incorrect fs sync behaviour.

This adds a sync_fs superblock operation for GFS2 and removes
the journal flush from write_super in favour of sync_fs where it
ought to be. This is more or less identical to the way in which ext3
does this.

This bug was pointed out by Russell Cattelan <cattelan@redhat.com>

Cc: Russell Cattelan <cattelan@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>