datagram: Make __skb_datagram_iter static

Fix sparse warning:

net/core/datagram.c:411:5: warning:
 symbol '__skb_datagram_iter' was not declared. Should it be static?
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/datagram.c

@@ -408,10 +408,10 @@ int skb_kill_datagram(struct sock *sk, struct sk_buff *skb, unsigned int flags)
 }
 EXPORT_SYMBOL(skb_kill_datagram);
 
-int __skb_datagram_iter(const struct sk_buff *skb, int offset,
-			struct iov_iter *to, int len, bool fault_short,
-			size_t (*cb)(const void *, size_t, void *, struct iov_iter *),
-			void *data)
+static int __skb_datagram_iter(const struct sk_buff *skb, int offset,
+			       struct iov_iter *to, int len, bool fault_short,
+			       size_t (*cb)(const void *, size_t, void *,
+					    struct iov_iter *), void *data)
 {
 	int start = skb_headlen(skb);
 	int i, copy = start - offset, start_off = offset, n;