rbd: return -ENOMEM instead of pool id if rbd_dev_create() fails

Returning pool id (i.e. >= 0) from a sysfs ->store() callback makes
userspace think it needs to retry the write.  Fix it - it's a leftover
from the times when the equivalent of rbd_dev_create() was the first
action in rbd_add().
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

drivers/block/rbd.c

@@ -5397,7 +5397,7 @@ static ssize_t do_rbd_add(struct bus_type *bus,
 	struct rbd_spec *spec = NULL;
 	struct rbd_client *rbdc;
 	bool read_only;
-	int rc = -ENOMEM;
+	int rc;
 
 	if (!try_module_get(THIS_MODULE))
 		return -ENODEV;
@@ -5432,8 +5432,10 @@ static ssize_t do_rbd_add(struct bus_type *bus,
 	}
 
 	rbd_dev = rbd_dev_create(rbdc, spec, rbd_opts);
-	if (!rbd_dev)
+	if (!rbd_dev) {
+		rc = -ENOMEM;
 		goto err_out_client;
+	}
 	rbdc = NULL;		/* rbd_dev now owns this */
 	spec = NULL;		/* rbd_dev now owns this */
 	rbd_opts = NULL;	/* rbd_dev now owns this */