UBI: make UBI devices dynamic

Hi,

here is the second iteration of the patch-set. Changes (were requested by Arnd):

* data_offset is removed as it makes no sens to set it to anything but the default value
* UBI control device is now registered as "misc" device, so we do not waste Linux major/minor numbers
* More commentaries in the ubi-user.h file

The ltree slab cache is not removed, it will be done separately.

The original message
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here is a patch-set which makes UBI devices dynamic. This means, that you may attach/detach MTD devices run-time, not just when UBI module is loaded/unloaded. This is very convenient - you do may compile UBI into the kernel and attach/detach needed MTD devices later, when your init scripts (loaded from initrd) decide which exactly MTD device(s) to attach. This also makes testing a lot easier.

The idea is similar to what the device mapper has: UBI creates an UBI control device, which registers itself in UBI sysfs hierarchy (/sys/class/ubi/ubi_ctrl) with dynamically allocated major and minor numbers. The control device has 2 ioctls - attach MTD device and detach MTD device.

This patch-set applies on top of my other UBI changes and fixes, which you may find in the UBI git.

I CC Frank and Andreas, as one of the main UBI users. I CC Arnd as an ioctl expert.

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Best regards,
Artem Bityutskiy (Битюцкий Артём)