mkfs.gfs2: Check for negative -j option
    
    When a bad -j argument like -j -2 is given, mkfs.gfs2 prints a confusing
    error that the fs will not fit on the disk. Add a specific check and
    error message for that case.
    
    Tidy up some other bad option error messages while we're at it.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>