IB/rdmavt: Correct qp_priv_alloc() return value test

The current drivers return errors from this calldown
wrapped in an ERR_PTR().

The rdmavt code incorrectly tests for NULL.

The code is fixed to use IS_ERR() and change ret according
to the driver return value.

Cc: Stable <stable@vger.kernel.org> # 4.6+
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>