RDMA/core: Fix error return in _ib_modify_qp()

Fix to return error code PTR_ERR() from the error handling case instead of
0.

Fixes: 51aab126 ("RDMA/core: Get xmit slave for LAG")
Link: https://lore.kernel.org/r/20201016075845.129562-1-jingxiangfeng@huawei.comSigned-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/core/verbs.c

@@ -1701,8 +1701,10 @@ static int _ib_modify_qp(struct ib_qp *qp, struct ib_qp_attr *attr,
 			slave = rdma_lag_get_ah_roce_slave(qp->device,
 							   &attr->ah_attr,
 							   GFP_KERNEL);
-			if (IS_ERR(slave))
+			if (IS_ERR(slave)) {
+				ret = PTR_ERR(slave);
 				goto out_av;
+			}
 			attr->xmit_slave = slave;
 		}
 	}