docs: 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