igb: add DCA support

igb: add DCA support

Add DCA support in the similar method that it was added to the ixgbe
driver recently. DCA allows the network device to put data in the
CPU cache and notify the chipset of that event. This reduces cache
misses during receives.

Signed-off-by: Jeb Cramer <cramerj@intel.com>
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>