sunrpc: fix UDP memory accounting

The commit f9b2ee714c5c ("SUNRPC: Move UDP receive data path
into a workqueue context"), as a side effect, moved the
skb_free_datagram() call outside the scope of the related socket
lock, but UDP sockets require such lock to be held for proper
memory accounting.
Fix it by replacing skb_free_datagram() with
skb_free_datagram_locked().

Fixes: f9b2ee714c5c ("SUNRPC: Move UDP receive data path into a workqueue context")
Reported-and-tested-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Cc: stable@vger.kernel.org # 4.4+
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

net/sunrpc/xprtsock.c

@@ -1074,7 +1074,7 @@ static void xs_udp_data_receive(struct sock_xprt *transport)
 		skb = skb_recv_datagram(sk, 0, 1, &err);
 		if (skb != NULL) {
 			xs_udp_data_read_skb(&transport->xprt, sk, skb);
-			skb_free_datagram(sk, skb);
+			skb_free_datagram_locked(sk, skb);
 			continue;
 		}
 		if (!test_and_clear_bit(XPRT_SOCK_DATA_READY, &transport->sock_state))