fix typo preventing ->mtu from being set.

Reference: /n/sources/patch/applied/readipifc-maxtu
Date: Mon Oct 16 17:53:23 CES 2006
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libip/readipifc.c	Mon Oct 16 17:52:49 2006
+++ /sys/src/libip/readipifc.c	Mon Oct 16 17:52:46 2006
@@ -94,7 +94,7 @@
 		free(ifc);
 		return l;
 	}
-	ifc->mtu = strtoul(findfield("maxmtu", f, n), nil, 10);
+	ifc->mtu = strtoul(findfield("maxtu", f, n), nil, 10);
 	ifc->sendra6 = atoi(findfield("sendra", f, n));
 	ifc->recvra6 = atoi(findfield("recvra", f, n));
 	ifc->rp.mflag = atoi(findfield("mflag", f, n));