[$] Recent improvements to BPF's struct_ops mechanism

Kui-Feng Lee spoke early in the BPF track at the 2024Linux Storage,Filesystem, Memory Management, and BPF Summit about some of therecent improvements to BPF. These changes were largely driven by thesched_ext work that David Vernet had covered inthe previous talk. Lee focused on changes relevant to struct_opsprograms, but several of those changes apply to all BPF programs.