for read capacity (10 (0x25) if we have more than
4 billion sectors, we must not return nsectors % (1<<32),
we must return 0xffffffff.

Reference: /n/atom/patch/applied2013/sd0x25overflow
Date: Tue Nov 12 16:45:06 CET 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/9/port/devsd.c	Tue Nov 12 16:42:38 2013
+++ /sys/src/9/port/devsd.c	Tue Nov 12 16:42:38 2013
@@ -1068,7 +1068,9 @@
 		 * Read capacity returns the LBA of the last sector.
 		 */
 		len = unit->sectors;
-		if(len > 0)
+		if(len >= 0xffffffff)
+			len = 0xffffffff;
+		else if(len > 0)
 			len--;
 		p = r->data;
 		*p++ = len>>24;
@@ -1624,10 +1626,12 @@
 	for(i = 0; sdifc[i] != nil; i++)
 		if(strcmp(sdifc[i]->name, cf->type) == 0)
 			break;
-	if(sdifc[i] == nil)
-		error("sd type not found");
-	if(p)
-		*(p-1) = '/';
+	if(sdifc[i] == nil){
+		snprint(buf, sizeof buf, "sd: type %s: not found", cf->type);
+		error(buf);
+	}
+	if(p != nil)
+		p[-1] = '/';
 
 	if(sdifc[i]->probe == nil)
 		error("sd type cannot probe");
--- /sys/src/nix/port/devsd.c	Tue Nov 12 16:42:38 2013
+++ /sys/src/nix/port/devsd.c	Tue Nov 12 16:42:38 2013
@@ -784,13 +784,13 @@
 	uvlong bno;
 
 	sdev = sdgetdev(DEV(c->qid));
+	if(sdev == nil)
+		error(Enonexist);
+	unit = sdev->unit[UNIT(c->qid)];
 	if(sdev == nil){
 		decref(&sdev->r);
 		error(Enonexist);
 	}
-	unit = sdev->unit[UNIT(c->qid)];
-	if(unit == nil)
-		error(Enonexist);
 
 	nchange = 0;
 	qlock(&unit->ctl);
@@ -1068,7 +1068,9 @@
 		 * Read capacity returns the LBA of the last sector.
 		 */
 		len = unit->sectors;
-		if(len > 0)
+		if(len >= 0xffffffff)
+			len = 0xffffffff;
+		else if(len > 0)
 			len--;
 		p = r->data;
 		*p++ = len>>24;