bpf: clean up unused-variable warning

The only user of this variable is inside of an #ifdef, causing
a warning without CONFIG_INET:

net/core/filter.c: In function '____bpf_sock_ops_cb_flags_set':
net/core/filter.c:3382:6: error: unused variable 'val' [-Werror=unused-variable]
  int val = argval & BPF_SOCK_OPS_ALL_CB_FLAGS;

This replaces the #ifdef with a nicer IS_ENABLED() check that
makes the code more readable and avoids the warning.

Fixes: b13d88072172 ("bpf: Adds field bpf_sock_ops_cb_flags to tcp_sock")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/core/filter.c

@@ -3381,17 +3381,13 @@ BPF_CALL_2(bpf_sock_ops_cb_flags_set, struct bpf_sock_ops_kern *, bpf_sock,
 	struct sock *sk = bpf_sock->sk;
 	int val = argval & BPF_SOCK_OPS_ALL_CB_FLAGS;
 
-	if (!sk_fullsock(sk))
+	if (!IS_ENABLED(CONFIG_INET) || !sk_fullsock(sk))
 		return -EINVAL;
 
-#ifdef CONFIG_INET
 	if (val)
 		tcp_sk(sk)->bpf_sock_ops_cb_flags = val;
 
 	return argval & (~BPF_SOCK_OPS_ALL_CB_FLAGS);
-#else
-	return -EINVAL;
-#endif
 }
 
 static const struct bpf_func_proto bpf_sock_ops_cb_flags_set_proto = {