tg3: Unwedge stuck MSI-X vectors

The previous patch changed the code so that new rx buffer submissions to
the hardware stall if a new submission would overwrite data needed by an
unserviced rx packet.  On very busy 5717 and 57765 asic rev devices,
there is a corner case where the hardware will fail to assert an MSI-X
interrupt for rx traffic.  If that vector's interrupt never has another
reason to assert, any rx buffers held will never be serviced.  If the
buffers are never serviced and the hardware consumes all the available
rx packets for other rx rings, deadlock will result.

The most reliable and least intrusive way to work around the problem is
to detect the case where new submissions would overwrite existing data
and force all rx interrupt vectors to fire.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tg3.h

@@ -2699,6 +2699,7 @@ struct tg3 {
 	struct net_device		*dev;
 	struct pci_dev			*pdev;
 
+	u32				coal_now;
 	u32				msg_enable;
 
 	/* begin "tx thread" cacheline section */