bpf: tcp: Limit calling some tcp cc functions to CONFIG_DYNAMIC_FTRACE

pahole currently only generates the btf_id for external function and
ftrace-able function.  Some functions in the bpf_tcp_ca_kfunc_ids
are static (e.g. cubictcp_init).  Thus, unless CONFIG_DYNAMIC_FTRACE
is set, btf_ids for those functions will not be generated and the
compilation fails during resolve_btfids.

This patch limits those functions to CONFIG_DYNAMIC_FTRACE.  I will
address the pahole generation in a followup and then remove the
CONFIG_DYNAMIC_FTRACE limitation.

Fixes: e78aea8b ("bpf: tcp: Put some tcp cong functions in allowlist for bpf-tcp-cc")
Reported-by: Cong Wang <xiyou.wangcong@gmail.com>
Reported-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210329221357.834438-1-kafai@fb.com

net/ipv4/bpf_tcp_ca.c

@@ -185,6 +185,7 @@ BTF_ID(func, tcp_reno_cong_avoid)
 BTF_ID(func, tcp_reno_undo_cwnd)
 BTF_ID(func, tcp_slow_start)
 BTF_ID(func, tcp_cong_avoid_ai)
+#ifdef CONFIG_DYNAMIC_FTRACE
 #if IS_BUILTIN(CONFIG_TCP_CONG_CUBIC)
 BTF_ID(func, cubictcp_init)
 BTF_ID(func, cubictcp_recalc_ssthresh)
@@ -211,6 +212,7 @@ BTF_ID(func, bbr_ssthresh)
 BTF_ID(func, bbr_min_tso_segs)
 BTF_ID(func, bbr_set_state)
 #endif
+#endif  /* CONFIG_DYNAMIC_FTRACE */
 BTF_SET_END(bpf_tcp_ca_kfunc_ids)
 
 static bool bpf_tcp_ca_check_kfunc_call(u32 kfunc_btf_id)