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ip/tcp: never raise the mss over the link mtu < 1280 for v6

v6 mandates minimum mtu of 1280, tho someone *could* setup
an interface with a lower mtu or set it lower for testing.

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

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

@@ -864,13 +864,14 @@

 	 * otherwise, we use the default MSS which assumes a

 	 * safe minimum MTU of 1280 bytes for V6.

 	 */  

-	if(r != nil && (version == V4 || (r->type & (Rifc|Runi)) != 0)){

+	if(r != nil){

 		ifc = r->ifc;

 		mtu = ifc->maxtu - ifc->m->hsize;

-		if(version == V6)

-			return mtu - (TCP6_PKT + TCP6_HDRSIZE);

-		else

+		if(version == V4)

 			return mtu - (TCP4_PKT + TCP4_HDRSIZE);

+		mtu -= TCP6_PKT + TCP6_HDRSIZE;

+		if((r->type & (Rifc|Runi)) != 0 || mtu <= DEF_MSS6)

+			return mtu;

 	}

 	if(version == V6)

 		return DEF_MSS6;