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>