l2tp: let pppol2tp_ioctl() fallback to dev_ioctl()

Return -ENOIOCTLCMD for unknown ioctl commands. This lets dev_ioctl()
handle generic socket ioctls like SIOCGIFNAME or SIOCGIFINDEX.
PF_PPPOX/PX_PROTO_OL2TP was one of the few socket types not honouring
this mechanism.
Signed-off-by: NGuillaume Nault <g.nault@alphalink.fr>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/l2tp/l2tp_ppp.c

@@ -1137,7 +1137,7 @@ static int pppol2tp_ioctl(struct socket *sock, unsigned int cmd,
 		break;
 
 	default:
-		return -ENOSYS;
+		return -ENOIOCTLCMD;
 	}
 
 	return 0;