kernel: handle bad blocks in ubi auto attach code

kernel: handle bad blocks in ubi auto attach code

The first block(s) of the ubi mtd device might be bad. We need to take
care on our own to skip the bad block(s) and read the next one(s).

Don't treat recoverable read errors as fatal and check for the UBI magic
if the data of a block could be recovered using ECC or similar.

Signed-off-by: Mathias Kresin <dev@kresin.me>
(backported from 0ac91d82ed4d077150db902eb12d383c2be44fb2)
(rebased patches)
Signed-off-by: Jo-Philipp Wich <jo@mein.io>