don't get confused by pread of directories from djc and the epinomous patch on sources

Reference: /n/atom/patch/applied/pread-offset
Date: Sun Jun 19 22:52:58 CES 2016
Signed-off-by: quanstro@quanstro.net

--- /sys/src/9/port/sysfile.c	Sun Jun 19 22:52:06 2016
+++ /sys/src/9/port/sysfile.c	Sun Jun 19 22:52:07 2016
@@ -680,10 +680,12 @@
 	}else
 		nnn = nn = devtab[c->type]->read(c, p, n, off);
 
-	lock(c);
-	c->devoffset += nn;
-	c->offset += nnn;
-	unlock(c);
+	if((c->qid.type & QTDIR) || offp == nil){
+		lock(c);
+		c->devoffset += nn;
+		c->offset += nnn;
+		unlock(c);
+	}
 
 	poperror();
 	cclose(c);