build failure (BTFIDS) with CONFIG_NET && !CONFIG_INET

[Michal Kubecek]

Hello,

our builds of s390x for zfcpdump fail since 5.9-rc1 with

    BTFIDS  vmlinux
  FAILED unresolved symbol tcp_timewait_sock
  make[1]: *** [/home/abuild/rpmbuild/BUILD/kernel-zfcpdump-5.9.rc7/linux-5.9-rc7/Makefile:1176: vmlinux] Error 255

I believe this is caused by commit fce557bcef11 ("bpf: Make btf_sock_ids
global") and the problem is caused by a specific configuration which has
CONFIG_NET enabled but CONFIG_INET disabled. IIUC there will be no user
of struct tcp_timewait_sock but btf_ids will try to generate BTF info
for it.

Michal