ulong is the incorrect cast; use uintptr.

Reference: /n/atom/patch/applied2013/fsarpcast
Date: Mon Aug 26 02:58:24 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/fs/ip/arp.c	Mon Aug 26 02:57:58 2013
+++ /sys/src/fs/ip/arp.c	Mon Aug 26 02:57:58 2013
@@ -429,7 +429,7 @@
 	mdsum = 0;
 
 	x = 0;
-	if((ulong)addr & 1) {
+	if((uintptr)addr & 1) {
 		if(len) {
 			hisum += addr[0];
 			len--;