[$] Reconsidering BPF ABI stability

The BPF subsystem exposes many aspects of the kernel's internal algorithmsand data structures; this naturally leads to concerns about maintaininginterface stability as the kernel changes. The longstanding position thatBPF offers no interface-stability guarantees to user space has alwaysseemed a little questionable; kernel developers have, in the past, foundthemselves having to maintain interfaces that were not intended to bestable. Now the BPF community is starting to think about what it mightmean to provide explicit stability promises for at least some of itsinterfaces.