bpf: Document cfi_stubs and owner fields in struct bpf_struct_ops

bpf: Document cfi_stubs and owner fields in struct bpf_struct_ops

Add missing kernel-doc documentation for the cfi_stubs and owner
fields in struct bpf_struct_ops to fix the following warnings:

  Warning: include/linux/bpf.h:1931 struct member 'cfi_stubs' not
  described in 'bpf_struct_ops'
  Warning: include/linux/bpf.h:1931 struct member 'owner' not
  described in 'bpf_struct_ops'

The cfi_stubs field was added in commit 2cd3e3772e41 ("x86/cfi,bpf:
Fix bpf_struct_ops CFI") to provide CFI stub functions for trampolines,
and the owner field is used for module reference counting.

Signed-off-by: Nirbhay Sharma <nirbhay.lkd@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20251120204620.59571-2-nirbhay.lkd@gmail.com