IB/hfi1: Show statistics counters under IB stats interface

Previously tools like hfi1stats had to access these counters through
debugfs, which often caused permission issue for non-root users. It is
not always acceptable to change the debugfs mounting permission due
to security concerns. When exposed under the IB stats interface, the
counters are universally readable by default.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Jianxin Xiong <jianxin.xiong@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>