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devip: less stupid hnputs_csum()

--- a/sys/src/9/ip/ipaux.c

+++ b/sys/src/9/ip/ipaux.c

@@ -556,16 +556,17 @@

 hnputs_csum(void *p, ushort v, uchar *pcsum)

 {

 	ulong csum;

+	ushort o;

+	csum = nhgets(pcsum)^0xFFFF;

+	o = nhgets(p);

+	hnputs(p, v);

 	if(((uchar*)p - pcsum) & 1){

-		hnputs_csum((uchar*)p-1, (nhgets((uchar*)p-1) & 0xFF00) | v>>8, pcsum);

-		hnputs_csum((uchar*)p+1, (nhgets((uchar*)p+1) & 0x00FF) | v<<8, pcsum);

-		return;

+		o = o << 8 | o >> 8;

+		v = v << 8 | v >> 8;

 	}

-	csum = nhgets(pcsum)^0xFFFF;

-	csum += nhgets(p)^0xFFFF;

+	csum += o ^ 0xFFFF;

 	csum += v;

-	hnputs(p, v);

 	while(v = csum >> 16)

 		csum = (csum & 0xFFFF) + v;

 	hnputs(pcsum, csum^0xFFFF);