[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>
diff --git a/net/core/sock.c b/net/core/sock.c
index c71b645..0ca0697 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1004,6 +1004,7 @@
 	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)
 {