The kernel devs went out of their way to label a whole bunch of kernel functions as GPL only. It's not about mainlining third party kernel modules, it's about not letting non-GPLed modules use certain functions in the kernel. This includes the fully open source OpenZFS project. It's not about CDDL / GPL incompatibilities. For OpenZFS it was some FPU context switching functions that had to be worked around when the kernel team labelled those functions as GPL only.