RDMA/cxgb4: Use vmalloc() for debugfs QP dump

This allows dumping thousands of QPs.  Log active open failures of
interest.

Signed-off-by: Vipul Pandya <vipul@chelsio.com>
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>