ubi_update_fastmap: could not find an early PEB

[Nikita V. Youshchenko]

> Especially the size of both pools used by fastmap needs to be discussed
> more. If the pool size is too small UBI was to write the fastmap very
> often. A too large pool means that attaching takes longer because
> fastmap has to rescan more PEBs.

I'd prefer to see those configurable.

I see two obvious cases when different settings are needed.

First is system that has very little changes at runtime. Maybe as small as 
save user settings if he ever changes those. Everything else is readonly. 
Then, better to have lower pool size to make attach faster.

Second is system with extensice writes expected. Then indeed pool needs to 
be larger to avoid too often fastmap writes.

Nikita