Bump `msr_batch_op.cpu` from `__u16` to `unsigned int`.

[rountree]

The mi300a nodes on Tioga have 192 cpus listed under /proc/cpuinfo. The kernel uses either int or unsigned int for nr_cpu_ids. We might not ever run into a machine node with more than 255 nodes, but I think this is worth bumping up proactively.