IB/qib: Modify software pma counters to use percpu variables

The counters, unicast_xmit, unicast_rcv, multicast_xmit, multicast_rcv
are now maintained as percpu variables.

The mad code is modified to add a z_ latch so that the percpu counters
monotonically increase with appropriate adjustments in the reset,
read logic to maintain the z_ latch.

This patch also corrects the fact the unitcast_xmit wasn't handled
at all for UC and RC QPs.

Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>