tcp: Add a stub for sk_defer_free_flush()

When compiling the kernel with CONFIG_INET disabled, the
sk_defer_free_flush() should be defined as a nop.

This resolves the following compilation error:
ld: net/core/sock.o: in function `sk_defer_free_flush':
./include/net/tcp.h:1378: undefined reference to `__sk_defer_free_flush'

Fixes: 79074a72d335 ("net: Flush deferred skb free on socket destroy")
Reported-by: kernel test robot <[e-mail]>
Reviewed-by: Tariq Toukan <[e-mail]>
Signed-off-by: Gal Pressman <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/tcp.h

···
 
 bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb);
 
+#ifdef CONFIG_INET
 void __sk_defer_free_flush(struct sock *sk);
 
 static inline void sk_defer_free_flush(struct sock *sk)
···
 		return;
 	__sk_defer_free_flush(sk);
 }
+#else
+static inline void sk_defer_free_flush(struct sock *sk) {}
+#endif
 
 int tcp_filter(struct sock *sk, struct sk_buff *skb);
 void tcp_set_state(struct sock *sk, int state);