block/rnbd: fix a null pointer dereference on dev->blk_symlink_name

Currently in the case where dev->blk_symlink_name fails to be allocates
the error return path attempts to set an end-of-string character to
the unallocated dev->blk_symlink_name causing a null pointer dereference
error. Fix this by returning with an explicity ENOMEM error (which also
is missing in the original code as was not initialized).

Fixes: 1eb54f8f ("block/rnbd: client: sysfs interface functions")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Addresses-Coverity: ("Dereference after null check")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/rnbd/rnbd-clt-sysfs.c

@@ -499,7 +499,7 @@ static int rnbd_clt_add_dev_symlink(struct rnbd_clt_dev *dev)
 	dev->blk_symlink_name = kzalloc(len, GFP_KERNEL);
 	if (!dev->blk_symlink_name) {
 		rnbd_clt_err(dev, "Failed to allocate memory for blk_symlink_name\n");
-		goto out_err;
+		return -ENOMEM;
 	}
 
 	ret = rnbd_clt_get_path_name(dev, dev->blk_symlink_name,