fstests: Add support for UBIFS

fstests: Add support for UBIFS

UBIFS is a filesystem for unmanaged flash memory devices. It works
on top of UBI (Unsorted Block Images) which is a wear leveling and
volume management layer on top of flash memory devices, which are
handled by the MTD subsystem (memory technology device).

Since the semantics of flash devices are drastically different from
regular block devices (blocks or "pages" must be erased before
writing, only larger groups of pages or "erase blocks" can be erased
at once, page write must be in order within an erase block, etc...)
it was decided to expose MTD devices as character devices with
ioctls for operations like erase.

Since erasing a flash erase block causes physical wear on the
device, eventually causing the erase blocks to go bad, the UBI layer
provides mainly transparent wear leveling on top of MTD devices. UBI
does not attempt to emulate a regular block device, but rather
something like a flash memory with idealized characteristics that
can be partitioned into multiple UBI volumes in a fashion somewhat
similar to LVM. UBI volumes are also exposed to user space as
character devices.

This patch mainly deals with some quirks of UBIFS like working on
top of character devices instead of block devices. Also UBIFS
automatically formats UBI devices when trying to mount an empty
device. The mkfs.ubifs program is mainly used for creating images.
This patch changes _scratch_mkfs and _scratch_mkfs_encrypted to
truncate the UBI volume instead, relying on the kernel to reformat
it on the next mount.

For _scratch_mkfs_encrypted this is actually required to get the
encryption tests to run, because mkfs.ubifs, at the time of writing
this, the kernel support for UBIFS encryption is fairly recent and
mkfs.ubifs does not have proper support yet.

The necessity of an additional -ubifs switch was discussed but auto
detection of UBIFS formated UBI devices could not be reproduced on
my end and is unlikely to work with empty UBI volumes anyway.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>