UBI: add volume id struct ubi_ainf_peb

This patch adds the volume id to struct ubi_ainf_peb when scanning the LEBs at
startup. PEBs now added to the erase queue will know their original LEB number
and volume id, if available, and will be -1 otherwise (for instance, if the VID
header is unreadable).

This was tested by creating an ubi device with 3 volumes and disabiling the
ubi_thread's do_work functionality. The different ubi volumes were formatted
to ubifs and had files created and erased.  The ubi modules was reloaded and
the list of LEB's added to the erased list was outputted, confirming the
volume ids and LEB numbers were appropriate.

Signed-off-by: Joel Reardon <reardonj@inf.ethz.ch>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>