udp: make function udp_skb_dtor_locked static

Function udp_skb_dtor_locked does not need to be in global scope
so make it static to fix sparse warning:

net/ipv4/udp.c: warning: symbol 'udp_skb_dtor_locked' was not
declared. Should it be static?

Fixes: 6dfb4367 ("udp: keep the sk_receive_queue held when splicing")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -1218,7 +1218,7 @@ void udp_skb_destructor(struct sock *sk, struct sk_buff *skb)
 EXPORT_SYMBOL(udp_skb_destructor);
 
 /* as above, but the caller held the rx queue lock, too */
-void udp_skb_dtor_locked(struct sock *sk, struct sk_buff *skb)
+static void udp_skb_dtor_locked(struct sock *sk, struct sk_buff *skb)
 {
 	udp_rmem_release(sk, skb->dev_scratch, 1, true);
 }