[$] 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 problem
plaguing the networking subsystem, and some potential ways to fix it. He works on
BPF programs that need to store socket-local data; amid other improvements to
the networking and BPF subsystems, retrieving that data has become a noticeable
bottleneck for his use case. His proposed fix prompted a good deal of discussion
about how the data should be laid out.