fs: mkdtemp shouldn't crash if no callback passed

As it is, `fs.mkdtemp` crashes with a C++ assertion if the callback
function is not passed. This patch uses `maybeCallback` to create one,
if no callback function is passed.

PR-URL: https://github.com/nodejs/node/pull/6828


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>