[NET]: Eliminate netif_rx massive packet drops.

Eliminate the throttling behaviour when the netif receive queue fills
because it behaves badly when using high speed networks under load.
The throttling cause multiple packet drops that cause TCP to go into
slow start mode. The same effective patch has been part of BIC TCP and
H-TCP as well as part of Web100.

The existing code drops 100's of packets when the queue fills;
this changes it to individual packet drop-tail. 

Signed-off-by: Stephen Hemmminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>