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kernel: support 32k iounit

32k iounit increases performance for 9p significantly,
both locally and remotely. This number isn't particularly
important, but it was picked because it's the largest
power of 2 smaller than qiomaxatomic, and because on
slower wifi it does not back things up too much.

--- a/sys/src/9/port/devmnt.c

+++ b/sys/src/9/port/devmnt.c

@@ -16,7 +16,8 @@

  * connection.

  */

-#define MAXRPC (IOHDRSZ+8192)

+#define MAXRPC (IOHDRSZ+32768)	/* maximum size of any RPC */

+#define MAXRPC0 (IOHDRSZ+8192)	/* maximum size of Tversion/Rversion pair */

 struct Mntrpc

 {

@@ -146,7 +147,7 @@

 	f.tag = NOTAG;

 	f.msize = msize;

 	f.version = v;

-	msg = malloc(8192+IOHDRSZ);

+	msg = malloc(MAXRPC0);

 	if(msg == nil)

 		exhausted("version memory");

 	if(waserror()){

@@ -153,7 +154,7 @@

 		free(msg);

 		nexterror();

 	}

-	k = convS2M(&f, msg, 8192+IOHDRSZ);

+	k = convS2M(&f, msg, MAXRPC0);

 	if(k == 0)

 		error("bad fversion conversion on send");

@@ -171,8 +172,8 @@

 	}

 	/* message sent; receive and decode reply */

-	for(k = 0; k < BIT32SZ || (k < GBIT32(msg) && k < 8192+IOHDRSZ); k += l){

-		l = devtab[c->type]->read(c, msg+k, 8192+IOHDRSZ-k, c->offset);

+	for(k = 0; k < BIT32SZ || (k < GBIT32(msg) && k < MAXRPC0); k += l){

+		l = devtab[c->type]->read(c, msg+k, MAXRPC0-k, c->offset);

 		if(l <= 0)

 			error("EOF receiving fversion reply");

 		lock(c);