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: 79074a72d3 ("net: Flush deferred skb free on socket destroy")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20220120123440.9088-1-gal@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/tcp.h

@@ -1369,6 +1369,7 @@ static inline bool tcp_checksum_complete(struct sk_buff *skb)
 
 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)
@@ -1377,6 +1378,9 @@ 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);