[$] BPF iterators for filesystems

In the first of two combined BPF and filesystem sessions at the2023 Linux Storage, Filesystem,Memory-Management and BPF Summit, Hou Tao introduced his BPF iteratorsfor filesystem information. Iterators forBPF are a relatively recent addition to the BPF landscape; they helpBPF programs step through kernel data structures in a loop-like manner, butwithout running afoul of the BPF verifier, which is notoriously hard toconvince about loops.