IB/qib: Add qp_stats debug file

This adds a seq_file iterator for reporting the QP hash table when the
qp_stats file is read.

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>