We do this implementation in several steps to not have the CI failing:
- first (this patch), we add struct_ops while keeping the existing infra
  available
- then we change the selftests, the examples and the existing in-tree
  HID-BPF programs
- then we remove the existing trace points making old HID-BPF obsolete

There are a few advantages of struct_ops over tracing:
- compatibility with sleepable programs (for hid_hw_raw_request() in
  a later patch)
- a lot simpler in the kernel: it's a simple rcu protected list
- we can add more parameters to the function called without much trouble
- the "attach" is now generic through BPF-core: the caller just needs to
  set hid_id and flags before calling __load().
- all the BPF tough part is not handled in BPF-core through generic
  processing
- hid_bpf_ctx is now only writable where it needs be
Acked-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20240608-hid_bpf_struct_ops-v3-3-6ac6ade58329@kernel.orgSigned-off-by: Benjamin Tissoires <bentiss@kernel.org>