bpf: Fix holes in special_kfunc_list if !CONFIG_NET

If the function is not available its entry has to be replaced with
BTF_ID_UNUSED instead of skipped.
Otherwise the list doesn't work correctly.

Reported-by: Alexei Starovoitov <[email protected]>
Closes: https://lore.kernel.org/lkml/CAADnVQJQpVziHzrPCCpGE5=8uzw2OkxP8gqe1FkJ6_XVVyVbNw@mail.gmail.com/
Fixes: 00a5acd ("bpf: Fix configuration-dependent BTF function references")
Signed-off-by: Thomas Weißschuh <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Link: https://lore.kernel.org/r/20241219-bpf-fix-special_kfunc_list-v1-1-d9d50dd61505@weissschuh.net
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: t-8ch

kernel/bpf/verifier.c

@@ -11739,6 +11739,9 @@ BTF_ID(func, bpf_rbtree_first)
 #ifdef CONFIG_NET
 BTF_ID(func, bpf_dynptr_from_skb)
 BTF_ID(func, bpf_dynptr_from_xdp)
+#else
+BTF_ID_UNUSED
+BTF_ID_UNUSED
 #endif
 BTF_ID(func, bpf_dynptr_slice)
 BTF_ID(func, bpf_dynptr_slice_rdwr)