IB/qib: Add percpu counter replacing qib_devdata int_counter

This patch replaces the dd->int_counter with a percpu counter.

The maintanance of qib_stats.sps_ints and int_counter are
combined into the new counter.

There are two new functions added to read the counter:
- qib_int_counter (for a particular qib_devdata)
- qib_sps_ints (for all HCAs)

A z_int_counter is added to allow the interrupt detection logic
to determine if interrupts have occured since z_int_counter
was "reset".

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>