xen/blkback: fix memory leaks

In read_per_ring_refs(), after 'req' and related memory regions are
allocated, xen_blkif_map() is invoked to map the shared frame, irq, and
etc. However, if this mapping process fails, no cleanup is performed,
leading to memory leaks. To fix this issue, invoke the cleanup before
returning the error.
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Wenwen Wang <wenwen@cs.uga.edu>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/xen-blkback/xenbus.c

@@ -965,6 +965,7 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
 		}
 	}
 
+	err = -ENOMEM;
 	for (i = 0; i < nr_grefs * XEN_BLKIF_REQS_PER_PAGE; i++) {
 		req = kzalloc(sizeof(*req), GFP_KERNEL);
 		if (!req)
@@ -987,7 +988,7 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
 	err = xen_blkif_map(ring, ring_ref, nr_grefs, evtchn);
 	if (err) {
 		xenbus_dev_fatal(dev, err, "mapping ring-ref port %u", evtchn);
-		return err;
+		goto fail;
 	}
 
 	return 0;
@@ -1007,8 +1008,7 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
 		}
 		kfree(req);
 	}
-	return -ENOMEM;
-
+	return err;
 }
 
 static int connect_ring(struct backend_info *be)