fix re-open failure issue when using gluster ioengine

Signed-off-by: rootfs <hchen@redhat.com>
Add unlink hook to ioengine API, gluster ioengine

fio would just call unlink even with engines that are not using the
operating systems file namespace... This provides a hook to allow
overriding that, with a default handler, and implements it for the
gluster ioengine.

There are others which it'd probably make sense I'm sure.

Huamin Chen looked over my changes to the gluster code earlier...
>I like this unlink idea, it would be great if you can also make unlink optional (if my coding reading is correct). This looks a great pull request candidate to fio. Please ping Axboe after you are done. He is not actively watching pull requests.

>Please also feel free to augment gluster code and pull me for review if necessary.

-castor

Signed-off-by: Jens Axboe <axboe@fb.com>
Update io engine version

Signed-off-by: Jens Axboe <axboe@fb.com>