[NET]: sk_release_kernel needs to be exported to modules

Fixes:

ERROR: "sk_release_kernel" [net/ipv6/ipv6.ko] undefined!
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/sock.c

@@ -1004,6 +1004,7 @@ void sk_release_kernel(struct sock *sk)
 	sk->sk_net = get_net(&init_net);
 	sock_put(sk);
 }
+EXPORT_SYMBOL(sk_release_kernel);
 
 struct sock *sk_clone(const struct sock *sk, const gfp_t priority)
 {