[MTD] UBI: Per volume update marker

[Artem Bityutskiy]

One more comment. Yo add a 'ubi_vtbl_updvol()' function which can only
change the update marker. But lets consider what you really need:

* set update marker
* clean update marker, set data length field (static-only, RAM-only),
clear the 'corrupted' flag (static-only, RAM-only).

Currently there is a 'ubi_vtbl_set_data_len()' function which sets data
length and clears the corrupted flag.

So I recommend:

* add a 'ubi_vtbl_set_upd_marker()' func - it can only set the
upd_marker flag.
* add a 'ubi_vtbl_clear_upd_marker()' func - it cleans the update
marker, set data length, and clears 'corrupted' flag.

And the 'UBI_VOL_UPD' and 'UBI_VOL_NOUPD' constant are not really
reasonable I guess - indeed, we have just a boolean flag - these
constants are overkill.

Thanks.

-- 
Best regards,
Artem Bityutskiy (Битюцкий Артём)