RDMA/cxgb4: debugfs files for dumping active stags

Add "stags" debugfs file.  This is useful for examining the TPTE and
PBL entries in adapter memory.  It allows scripts to dump just the
active entries.

Also clean up the "qps" file handlers and shared common code.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>