in devmnt.c replace hard-coded numbers IOHDRSZ+8192 with MAXRPC for easy tweaking

Reference: /n/sources/patch/mnt-maxrpc
Date: Fri Apr  4 10:48:19 CES 2014
Signed-off-by: yarikos@gmail.com

--- /sys/src/9/port/devmnt.c	Fri Apr  4 10:47:55 2014
+++ /sys/src/9/port/devmnt.c	Fri Apr  4 10:47:43 2014
@@ -150,14 +150,14 @@
 	f.tag = NOTAG;
 	f.msize = msize;
 	f.version = v;
-	msg = malloc(8192+IOHDRSZ);
+	msg = malloc(MAXRPC);
 	if(msg == nil)
 		exhausted("version memory");
 	if(waserror()){
 		free(msg);
 		nexterror();
 	}
-	k = convS2M(&f, msg, 8192+IOHDRSZ);
+	k = convS2M(&f, msg, MAXRPC);
 	if(k == 0)
 		error("bad fversion conversion on send");
 
@@ -176,7 +176,7 @@
 	}
 
 	/* message sent; receive and decode reply */
-	k = devtab[c->type]->read(c, msg, 8192+IOHDRSZ, c->offset);
+	k = devtab[c->type]->read(c, msg, MAXRPC, c->offset);
 	if(k <= 0)
 		error("EOF receiving fversion reply");