I'm not familiar with the kernel RNG specifically, but I think the hash function is only used with small inputs and outputs, and it's the stream cipher (ChaCha)

I'm not familiar with the kernel RNG specifically, but I think the hash function is only used with small inputs and outputs, and it's the stream cipher (ChaCha) that has the more performance-sensitive job of generating lots of output bytes. So the performance differences between BLAKE2s and BLAKE3 probably aren't very important here, and the fact that a BLAKE2s implementation is already in the kernel makes this an easy change.