Are you sure? I don't think the kernel even compiles with a C++ compiler [1]. Also, I just ran cloc on my linux repo and didn't get any actual C++ code.
Perhaps some external kernel modules are written in C++, but that seems like a bad idea too [2].
That [1] was what I remembered. They were preparing for C++. But obviously it didn't get as far as actually including C++ code in the mainline kernel. Maybe it was a preparation in order to allow experimenting with C++ in the kernel.