[$] The kernel "closure" API

The data structure known as a "closure" first found its way into themainline kernel with the addition of bcache in the 3.10 developmentcycle. With the advent of bcachefs in6.7, though, it acquired a second user and was moved to the kernel'slib directory, making it available to other kernel users as well.The documentation of closures in the source is better than that of manythings in the kernel, but there is still room for a gentler introduction.