skmsg: Make sk_psock_destroy() static

sk_psock_destroy() is a RCU callback, I can't see any reason why
it could be used outside.

Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Jakub Sitnicki <jakub@cloudflare.com>
Cc: Lorenz Bauer <lmb@cloudflare.com>
Link: https://lore.kernel.org/bpf/20210127221501.46866-1-xiyou.wangcong@gmail.com

include/linux/skmsg.h

@@ -390,7 +390,6 @@ static inline struct sk_psock *sk_psock_get(struct sock *sk)
 }
 
 void sk_psock_stop(struct sock *sk, struct sk_psock *psock);
-void sk_psock_destroy(struct rcu_head *rcu);
 void sk_psock_drop(struct sock *sk, struct sk_psock *psock);
 
 static inline void sk_psock_put(struct sock *sk, struct sk_psock *psock)

net/core/skmsg.c

@@ -669,14 +669,13 @@ static void sk_psock_destroy_deferred(struct work_struct *gc)
 	kfree(psock);
 }
 
-void sk_psock_destroy(struct rcu_head *rcu)
+static void sk_psock_destroy(struct rcu_head *rcu)
 {
 	struct sk_psock *psock = container_of(rcu, struct sk_psock, rcu);
 
 	INIT_WORK(&psock->gc, sk_psock_destroy_deferred);
 	schedule_work(&psock->gc);
 }
-EXPORT_SYMBOL_GPL(sk_psock_destroy);
 
 void sk_psock_drop(struct sock *sk, struct sk_psock *psock)
 {