UBI: R/O block driver on top of UBI volumes

This commit introduces read-only block device emulation on top of UBI volumes.

Given UBI takes care of wear leveling and bad block management it's possible
to add a thin layer to enable block device access to UBI volumes.
This allows to use a block-oriented filesystem on a flash device.

The UBI block devices are meant to be used in conjunction with any
regular, block-oriented file system (e.g. ext4), although it's primarily
targeted at read-only file systems, such as squashfs.

Block devices are created upon user request through new ioctls:
UBI_IOCVOLATTBLK to attach and UBI_IOCVOLDETBLK to detach.
Also, a new UBI module parameter is added 'ubi.block'. This parameter is
needed in order to attach a block device on boot-up time, allowing to
mount the rootfs on a ubiblock device.
For instance, you could have these kernel parameters:

  ubi.mtd=5 ubi.block=0,0 root=/dev/ubiblock0_0

Or, if you compile ubi as a module:

  $ modprobe ubi mtd=/dev/mtd5 block=/dev/ubi0_0

Artem: amend commentaries and massage the patch a little bit.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

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