iw_cxgb4: don't block in destroy_qp awaiting the last deref

Blocking in c4iw_destroy_qp() causes a deadlock when apps destroy a qp
or disconnect a cm_id from their cm event handler function.  There is
no need to block here anyway, so just replace the refcnt atomic with a
kref object and free the memory on the last put.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>