IB/hfi1: Add running average for adaptive pio

The adaptive PIO implementation only considers the current packet size
when deciding between SDMA and pio for a packet.

This causes credit return forces if small and large packets are
interleaved.

Add a running average to avoid costly credit forces so that a large
sequence of small packets is required to go below the threshold that
chooses pio.

Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>