bpf, sk_msg: Don't clear saved sock proto on restore

There is no need to clear psock->sk_proto when restoring socket protocol
callbacks in sk->sk_prot. The psock is about to get detached from the sock
and eventually destroyed. At worst we will restore the protocol callbacks
and the write callback twice.

This makes reasoning about psock state easier. Once psock is initialized,
we can count on psock->sk_proto always being set.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200217121530.754315-3-jakub@cloudflare.com

include/linux/skmsg.h

@@ -359,13 +359,7 @@ static inline void sk_psock_restore_proto(struct sock *sk,
 					  struct sk_psock *psock)
 {
 	sk->sk_prot->unhash = psock->saved_unhash;
-
-	if (psock->sk_proto) {
-		tcp_update_ulp(sk, psock->sk_proto, psock->saved_write_space);
-		psock->sk_proto = NULL;
-	} else {
-		sk->sk_write_space = psock->saved_write_space;
-	}
+	tcp_update_ulp(sk, psock->sk_proto, psock->saved_write_space);
 }
 
 static inline void sk_psock_set_state(struct sk_psock *psock,