mkfs.gfs2: Add a root_inherit_jdata extended option
    
    Add an extended option to allow the GFS2_DIF_INHERIT_JDATA flag to be
    set on the root dinode at mkfs time.
    
    This option is only to be used for testing so it is not documented.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>