dax_io(): don't let non-error value escape via retval instead of EFAULT

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Cc: stable@vger.kernel.org # 4.0+
Signed-off-by: Jens Axboe <axboe@fb.com>

fs/dax.c

@@ -169,8 +169,10 @@ static ssize_t dax_io(struct inode *inode, struct iov_iter *iter,
 		else
 			len = iov_iter_zero(max - pos, iter);
 
-		if (!len)
+		if (!len) {
+			retval = -EFAULT;
 			break;
+		}
 
 		pos += len;
 		addr += len;