ipc: use simple_read_from_buffer()

Also this patch kills unneccesary trailing NULL character.
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Nadia Derbey <Nadia.Derbey@bull.net>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Pierre Peiffer <peifferp@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/mqueue.c

@@ -314,15 +314,11 @@ static int mqueue_unlink(struct inode *dir, struct dentry *dentry)
 *	through std routines)
 */
 static ssize_t mqueue_read_file(struct file *filp, char __user *u_data,
-				size_t count, loff_t * off)
+				size_t count, loff_t *off)
 {
 	struct mqueue_inode_info *info = MQUEUE_I(filp->f_path.dentry->d_inode);
 	char buffer[FILENT_SIZE];
-	size_t slen;
-	loff_t o;
-
-	if (!count)
-		return 0;
+	ssize_t ret;
 
 	spin_lock(&info->lock);
 	snprintf(buffer, sizeof(buffer),
@@ -335,21 +331,14 @@ static ssize_t mqueue_read_file(struct file *filp, char __user *u_data,
 			pid_vnr(info->notify_owner));
 	spin_unlock(&info->lock);
 	buffer[sizeof(buffer)-1] = '\0';
-	slen = strlen(buffer)+1;
-
-	o = *off;
-	if (o > slen)
-		return 0;
-
-	if (o + count > slen)
-		count = slen - o;
 
-	if (copy_to_user(u_data, buffer + o, count))
-		return -EFAULT;
+	ret = simple_read_from_buffer(u_data, count, off, buffer,
+				strlen(buffer));
+	if (ret <= 0)
+		return ret;
 
-	*off = o + count;
 	filp->f_path.dentry->d_inode->i_atime = filp->f_path.dentry->d_inode->i_ctime = CURRENT_TIME;
-	return count;
+	return ret;
 }
 
 static int mqueue_flush_file(struct file *filp, fl_owner_t id)