CMSG_DATA should return an unsigned char*.

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_socket.h.html
> CMSG_DATA(cmsg)
>    If the argument is a pointer to a cmsghdr structure, this macro
>    shall return an unsigned character pointer to the data array
>    associated with the cmsghdr structure.

Change-Id: I3f89ba19cbca4e6727abc65a2bbcd59267892ba8