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>