Return length of message payload including user header.

    https://github.com/thom311/libnl/blob/libnl3_2_25/lib/genl/genl.c#L224

    Positional arguments:
    gnlh -- Generic Netlink message header (genlmsghdr class instance).

    Returns:
    Length of user payload including an eventual user header in number of bytes.

def genlmsg_len(gnlh):
    """Return length of message payload including user header.

    https://github.com/thom311/libnl/blob/libnl3_2_25/lib/genl/genl.c#L224

    Positional arguments:
    gnlh -- Generic Netlink message header (genlmsghdr class instance).

    Returns:
    Length of user payload including an eventual user header in number of bytes.
    """
    nlh = nlmsghdr(bytearray_ptr(gnlh.bytearray, -NLMSG_HDRLEN, oob=True))
    return nlh.nlmsg_len - GENL_HDRLEN - NLMSG_HDRLEN