balloon: fix up comments

Lots of comments bitrotted. Fix them up.

Fixes: 418a3ab1 (mm/balloon_compaction: List interfaces)
Reviewed-by: Wei Wang <wei.w.wang@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Acked-by: Nadav Amit <namit@vmware.com>