[PATCH RFC v2] UBI: New ioctl() to support ubidump

[hujianyang]

> 
> Isn't this racy?
> i.e. If a LEB change happens between ubi_is_mapped() and "pnum =
> vol->eba_tbl[req.lnum]".
> 

As I was writing this code, I used this ubi_is_mapped() to make sure
lnum is less than @vol->reserved_pebs and !@vol->upd_marker in order
to avoid kernel panic when we get pnum from eba_tbl.

Even we locked this ioctl, peb may also be changed before we read it
from MTD. That's why I didn't focus on this race. We can use some
condition check to make sure this vol is not mounted but user may use
this utility on rootfs.

Bill showed me a new method to get pnum without UBI driver. I will
try that first but this new method can't avoid peb changes before
we read it either. We should consider more on it.

Thanks!

Hu