tcp: Remove timewait_sock_ops.twsk_destructor().

Since DCCP has been removed, sk->sk_prot->twsk_prot->twsk_destructor
is always tcp_twsk_destructor().

Let's call tcp_twsk_destructor() directly in inet_twsk_free() and
remove ->twsk_destructor().

While at it, tcp_twsk_destructor() is un-exported.

Signed-off-by: Kuniyuki Iwashima <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/timewait_sock.h

···
 	struct kmem_cache	*twsk_slab;
 	char		*twsk_slab_name;
 	unsigned int	twsk_obj_size;
-	void		(*twsk_destructor)(struct sock *sk);
 };
-
-static inline void twsk_destructor(struct sock *sk)
-{
-	if (sk->sk_prot->twsk_prot->twsk_destructor != NULL)
-		sk->sk_prot->twsk_prot->twsk_destructor(sk);
-}
 
 #endif /* _TIMEWAIT_SOCK_H */

net/ipv4/inet_timewait_sock.c

···
 #include <net/inet_hashtables.h>
 #include <net/inet_timewait_sock.h>
 #include <net/ip.h>
-
+#include <net/tcp.h>
 
 /**
  *	inet_twsk_bind_unhash - unhash a timewait socket from bind hash
···
 void inet_twsk_free(struct inet_timewait_sock *tw)
 {
 	struct module *owner = tw->tw_prot->owner;
-	twsk_destructor((struct sock *)tw);
+
+	tcp_twsk_destructor((struct sock *)tw);
 	kmem_cache_free(tw->tw_prot->twsk_prot->twsk_slab, tw);
 	module_put(owner);
 }

net/ipv4/tcp_ipv4.c

···
 
 static struct timewait_sock_ops tcp_timewait_sock_ops = {
 	.twsk_obj_size	= sizeof(struct tcp_timewait_sock),
-	.twsk_destructor= tcp_twsk_destructor,
 };
 
 void inet_sk_rx_dst_set(struct sock *sk, const struct sk_buff *skb)

net/ipv4/tcp_minisocks.c

···
 #endif
 	tcp_ao_destroy_sock(sk, true);
 }
-EXPORT_IPV6_MOD_GPL(tcp_twsk_destructor);
 
 void tcp_twsk_purge(struct list_head *net_exit_list)
 {

net/ipv6/tcp_ipv6.c

···
 
 static struct timewait_sock_ops tcp6_timewait_sock_ops = {
 	.twsk_obj_size	= sizeof(struct tcp6_timewait_sock),
-	.twsk_destructor = tcp_twsk_destructor,
 };
 
 INDIRECT_CALLABLE_SCOPE void tcp_v6_send_check(struct sock *sk, struct sk_buff *skb)