<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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