ixgbe: Introduce adaptive interrupt moderation

82598 can produce a formidable interrupt rate, and is largely
unusable without some form of moderation. The default behaviour
before this patch is to limit irq's to a reasonable number.
However, just like our other drivers we can reduce latency
for small packet-type traffic considerably by allowing the
irq rate to go up dynamically.

This patch introduces a simple irq moderation algorithm based
on traffic analysis. The driver will use more CPU to service
small packets quicker but will perform the same on bulk traffic
as the old code.

Signed-off-by: Ayyappan Veeraiyan <ayyappan.veeraiyan@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>