[$] Inline socket-local storage for BPF

Martin Lau gave a talk in the BPF track of the 2025 Linux Storage, Filesystem,Memory-Management, and BPF Summit about a performance problemplaguing the networking subsystem, and some potential ways to fix it. He works onBPF programs that need to store socket-local data; amid other improvements tothe networking and BPF subsystems, retrieving that data has become a noticeablebottleneck for his use case. His proposed fix prompted a good deal of discussionabout how the data should be laid out.