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>
diff --git a/include/rdma/rdma_vt.h b/include/rdma/rdma_vt.h
index 16274e2..9c9a27d 100644
--- a/include/rdma/rdma_vt.h
+++ b/include/rdma/rdma_vt.h
@@ -203,7 +203,9 @@
 
 	/*
 	 * Allocate a private queue pair data structure for driver specific
-	 * information which is opaque to rdmavt.
+	 * information which is opaque to rdmavt.  Errors are returned via
+	 * ERR_PTR(err).  The driver is free to return NULL or a valid
+	 * pointer.
 	 */
 	void * (*qp_priv_alloc)(struct rvt_dev_info *rdi, struct rvt_qp *qp,
 				gfp_t gfp);