The problem was that I was updating the head register multiple times in the
rcvhdrq processing loop, and setting the counter on each update.  Since that
meant that the tail register was ahead of head for all but the last update, we
would get extra interrupts.  The fix was to not write the counter value except
on the last update.

I also changed to update rcvhdrhead and rcvegrindexhead at most every 16
packets, if there were lots of packets in the queue (and of course, on the
last packet, regardless).

I also made some small cleanups while debugging this.

With these changes, xeon/monty typically sees two openib packets per interrupt
on sdp and ipoib, opteron/monty is about 1.25 pkts/intr.

I'm seeing about 3800 Mbit/s monty/xeon, and 5000-5100 opteron/monty with
netperf sdp.  Netpipe doesn't show as good as that, peaking at about 4400 on
opteron/monty sdp.  Plain ipoib xeon is about 2100+ netperf, opteron 2900+, at
128KB

Signed-off-by:  <olson@eng-12.pathscale.com>
Signed-off-by: Bryan O'Sullivan <bos@pathscale.com>
Cc: "Michael S. Tsirkin" <mst@mellanox.co.il>
Cc: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>