/dev/zero: avoid repeated access_ok() checks

In read_zero, we check for access_ok() once for the count bytes.  It is
unnecessarily checked again in clear_user.  Use __clear_user, which does
not check for access_ok().
Signed-off-by: NNikanth Karthikesan <knikanth@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/char/mem.c

@@ -690,7 +690,7 @@ static ssize_t read_zero(struct file * file, char __user * buf,
 
 		if (chunk > PAGE_SIZE)
 			chunk = PAGE_SIZE;	/* Just for latency reasons */
-		unwritten = clear_user(buf, chunk);
+		unwritten = __clear_user(buf, chunk);
 		written += chunk - unwritten;
 		if (unwritten)
 			break;