[$] Better linked-list traversal in BPF

Before loading a BPF program, the kernel must verify that the program issafe to run; among other things, that verification includes ensuring thatthe program will terminate within a bounded time. That requirement haslong made writing loops in BPF a challenging task. The situation hasimproved over the years for some types of loops, but others - includinglinked-list traversal - are still awkward in BPF programs. A new set ofBPF primitives aims to make life easier for this use case through theinstallation of what can be seen as a sort of circuit breaker.