[$] Documenting counted-by relationships in kernel data structures

The C language is expressive in many ways, but it still does not have waysto express many of the relationships between fields in a data structure.That gap can be at least partially filled, though, if one is willing tocreate and use non-standard extensions. The adoption of of thoseextensions, in the form of the __counted_by() macro, has beenmerged for the 6.5 kernel release, even though the compiler feature itdepends on has not yet been finalized.