net: use indirect call helpers for sk->sk_prot->release_cb()

When adding sk->sk_prot->release_cb() call from __sk_flush_backlog()
Paolo suggested using indirect call helpers to take care of
CONFIG_RETPOLINE=y case.

It turns out Google had such mitigation for years in release_sock(),
it is time to make this public :)

Suggested-by: Paolo Abeni <[e-mail]>
Signed-off-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/sock.c

···
 	__release_sock(sk);
 
 	if (sk->sk_prot->release_cb)
-		sk->sk_prot->release_cb(sk);
+		INDIRECT_CALL_INET_1(sk->sk_prot->release_cb,
+				     tcp_release_cb, sk);
+
 	spin_unlock_bh(&sk->sk_lock.slock);
 }
 EXPORT_SYMBOL_GPL(__sk_flush_backlog);
···
 		__release_sock(sk);
 
 	if (sk->sk_prot->release_cb)
-		sk->sk_prot->release_cb(sk);
+		INDIRECT_CALL_INET_1(sk->sk_prot->release_cb,
+				     tcp_release_cb, sk);
 
 	sock_release_ownership(sk);
 	if (waitqueue_active(&sk->sk_lock.wq))