IB/qib: Add per-context stats interface

This patch adds a debugfs stats interface for per kernel contexts
packet counts.

The code uses the opcode stats count and eliminates the counter in the
context.

Reviewed-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>