af_packet / TX_RING not fully non-blocking (w/ MSG_DONTWAIT).

This patch fixes an issue where the send(MSG_DONTWAIT) call
on a TX_RING is not fully non-blocking in cases where the device's sndBuf is
full. We pass nonblock=true to sock_alloc_send_skb() and return any possibly
occuring error code (most likely EGAIN) to the caller. As the fast-path stays
as it is, we keep the unlikely() around skb == NULL.

Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -2311,11 +2311,14 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 		tlen = dev->needed_tailroom;
 		skb = sock_alloc_send_skb(&po->sk,
 				hlen + tlen + sizeof(struct sockaddr_ll),
-				0, &err);
+				!need_wait, &err);
 
-		if (unlikely(skb == NULL))
+		if (unlikely(skb == NULL)) {
+			/* we assume the socket was initially writeable ... */
+			if (likely(len_sum > 0))
+				err = len_sum;
 			goto out_status;
-
+		}
 		tp_len = tpacket_fill_skb(po, skb, ph, dev, size_max, proto,
 					  addr, hlen);
 		if (tp_len > dev->mtu + dev->hard_header_len) {