UBIFS Panic

[hujianyang]

> 
> Is enabling UBI_FASTMAP the cause for the panic? If so I can disabled the
> feature and re-test. Do you see any compatibility issue going from:
> Current config -> New config -> Failsafe config
> 

I don't know. But I found func ubi_wl_get_peb() in drivers/mtd/ubi/wl.c
will return an error ENOSPC without any error messages. And this func
seems related with fastmap feature.

I don't have much experience in this.