Initiating the freeze from kernelspace doesn't make much sense. With virtio we could add in-band freeze request to the protocol, and although that would be a major change in that way virtio-blk works right now it's at least doable. But all other "real" storage targets only communicate with their initators over out of band procotols that are entirely handled in userspace, and given their high-level nature better are - that is if we know them at all given how vendors like to keep this secrete IP closed and just offer userspace management tools in binary form.
building new infrastructure in the kernel just for virtio, while needing to duplicate the same thing in userspace for all real storage seems like a really bad idea. That is in addition to the userspace freeze notifier similar to what e.g. Windows has - if the freeze process is driven from userspace it's much easier to handle those properly compared to requiring kernel upcalls.
