RDMA/srpt: Fix disabling device management

Avoid disabling device management for devices that don't support
Management datagrams (MADs) by checking if the "mad_agent" pointer is
initialized before calling ib_modify_port, also fix the error flow in
srpt_refresh_port() to disable device management if
ib_register_mad_agent() fail.

Fixes: 09f8a14 ("RDMA/srpt: Fix handling of SR-IOV and iWARP ports")
Link: https://lore.kernel.org/r/20200514114720.141139-1-kamalheib1@gmail.com
Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/ulp/srpt/ib_srpt.c

@@ -607,6 +607,11 @@ static int srpt_refresh_port(struct srpt_port *sport)
 			       dev_name(&sport->sdev->device->dev), sport->port,
 			       PTR_ERR(sport->mad_agent));
 			sport->mad_agent = NULL;
+			memset(&port_modify, 0, sizeof(port_modify));
+			port_modify.clr_port_cap_mask = IB_PORT_DEVICE_MGMT_SUP;
+			ib_modify_port(sport->sdev->device, sport->port, 0,
+				       &port_modify);
+
 		}
 	}
 
@@ -630,9 +635,8 @@ static void srpt_unregister_mad_agent(struct srpt_device *sdev)
 	for (i = 1; i <= sdev->device->phys_port_cnt; i++) {
 		sport = &sdev->port[i - 1];
 		WARN_ON(sport->port != i);
-		if (ib_modify_port(sdev->device, i, 0, &port_modify) < 0)
-			pr_err("disabling MAD processing failed.\n");
 		if (sport->mad_agent) {
+			ib_modify_port(sdev->device, i, 0, &port_modify);
 			ib_unregister_mad_agent(sport->mad_agent);
 			sport->mad_agent = NULL;
 		}