if we use pread/write we don't need to protect the kernel's
file pointer with a qlock, and that code can go away.

Reference: /n/atom/patch/applied2013/kfspwrite
Date: Fri Oct 25 03:13:25 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/disk/kfs/devwren.c	Fri Oct 25 03:12:44 2013
+++ /sys/src/cmd/disk/kfs/devwren.c	Fri Oct 25 03:12:44 2013
@@ -9,7 +9,6 @@
 typedef struct Wren	Wren;
 
 struct Wren{
-	QLock;
 	Device	dev;
 	uvlong	size;
 	int	fd;
@@ -49,8 +48,7 @@
 		panic("can't open %s", wrenfile);
 	if((d = dirfstat(fd)) == nil)
 		panic("can't stat %s\n", wrenfile);
-	seek(fd, 0, 0);
-	i = read(fd, buf, sizeof buf);
+	i = pread(fd, buf, sizeof buf, 0);
 	if(i < sizeof buf)
 		panic("can't read %s", wrenfile);
 	badmagic = 0;
@@ -87,9 +85,7 @@
 	fd = w->fd;
 	memset(buf, 0, sizeof buf);
 	sprint(buf+256, "%s%d\n", WMAGIC, RBUFSIZE);
-	qlock(w);
-	i = seek(fd, 0, 0) < 0 || write(fd, buf, RBUFSIZE) != RBUFSIZE;
-	qunlock(w);
+	i = pwrite(fd, buf, RBUFSIZE, 0) != RBUFSIZE;
 	if(i < 0)
 		panic("can't ream disk");
 }
@@ -149,11 +145,9 @@
 
 	w = wren(dev);
 	fd = w->fd;
-	qlock(w);
-	i = seek(fd, (vlong)addr*RBUFSIZE, 0) == -1 || read(fd, b, RBUFSIZE) != RBUFSIZE;
-	qunlock(w);
+	i = pread(fd, b, RBUFSIZE, (vlong)addr*RBUFSIZE) != RBUFSIZE;
 	if(i)
-		print("wrenread failed: %r\n");
+		print("wrenread failed byte %lld: %r\n", (vlong)addr*RBUFSIZE);
 	return i;
 }
 
@@ -165,10 +159,8 @@
 
 	w = wren(dev);
 	fd = w->fd;
-	qlock(w);
-	i = seek(fd, (vlong)addr*RBUFSIZE, 0) == -1 || write(fd, b, RBUFSIZE) != RBUFSIZE;
-	qunlock(w);
+	i = pwrite(fd, b, RBUFSIZE, (vlong)addr*RBUFSIZE) != RBUFSIZE;
 	if(i)
-		print("wrenwrite failed: %r\n");
+		print("wrenwrite failed byte %lld: %r\n", (vlong)addr*RBUFSIZE);
 	return i;
 }