appletalk: Set error code if register_snap_client failed

mainline inclusion
from mainline-v5.1
commit c93ad1337ad06a718890a89cdd85188ff9a5a5cc
category: bugfix
bugzilla: 13690
CVE: CVE-2019-19227

-------------------------------------------------

If register_snap_client fails in atalk_init,
error code should be set, otherwise it will
triggers NULL pointer dereference while unloading
module.

Fixes: 9804501fa122 ("appletalk: Fix potential NULL pointer dereference in unregister_snap_client")
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Reviewed-by: NYue Haibing <yuehaibing@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

net/appletalk/ddp.c

@@ -1925,6 +1925,7 @@ static int __init atalk_init(void)
 	ddp_dl = register_snap_client(ddp_snap_id, atalk_rcv);
 	if (!ddp_dl) {
 		pr_crit("Unable to register DDP with SNAP.\n");
+		rc = -ENOMEM;
 		goto out_sock;
 	}