net/packet: implement shortcut in tpacket_rcv()

tpacket_rcv() can be hit under DDOS quite hard, since
it will always grab a socket spinlock, to eventually find
there is no room for an additional packet.

Using tcpdump [1] on a busy host can lead to catastrophic consequences,
because of all cpus spinning on a contended spinlock.

This replicates a similar strategy used in packet_rcv()

[1] Also some applications mistakenly use af_packet socket
bound to ETH_P_ALL only to send packets.
Receive queue is never drained and immediately full.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -2193,6 +2193,12 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 	if (!res)
 		goto drop_n_restore;
 
+	/* If we are flooded, just give up */
+	if (__packet_rcv_has_room(po, skb) == ROOM_NONE) {
+		atomic_inc(&po->tp_drops);
+		goto drop_n_restore;
+	}
+
 	if (skb->ip_summed == CHECKSUM_PARTIAL)
 		status |= TP_STATUS_CSUMNOTREADY;
 	else if (skb->pkt_type != PACKET_OUTGOING &&