Head of attributes data.

    https://github.com/thom311/libnl/blob/libnl3_2_25/lib/msg.c#L143

    Positional arguments:
    nlh -- Netlink message header (nlmsghdr class instance).
    hdrlen -- length of family specific header (integer).

    Returns:
    First attribute (nlattr class instance with others in its payload).

def nlmsg_attrdata(nlh, hdrlen):
    """Head of attributes data.

    https://github.com/thom311/libnl/blob/libnl3_2_25/lib/msg.c#L143

    Positional arguments:
    nlh -- Netlink message header (nlmsghdr class instance).
    hdrlen -- length of family specific header (integer).

    Returns:
    First attribute (nlattr class instance with others in its payload).
    """
    data = nlmsg_data(nlh)
    return libnl.linux_private.netlink.nlattr(bytearray_ptr(data, libnl.linux_private.netlink.NLMSG_ALIGN(hdrlen)))