Defragment only one BTRFS mountpoint.

def _defragment_mountpoint(mountpoint):
    '''
    Defragment only one BTRFS mountpoint.
    '''
    out = __salt__['cmd.run_all']("btrfs filesystem defragment -f {0}".format(mountpoint))
    return {
        'mount_point': mountpoint,
        'passed': not out['stderr'],
        'log': out['stderr'] or False,
        'range': False,
    }