ubi: Add debugfs file for tracking PEB state

Add a file under debugfs to allow easy access to the erase count for
each physical erase block on an UBI device.  This is useful when
debugging data integrity issues with UBIFS on NAND flash devices.

Signed-off-by: Ben Shelton <ben.shelton@ni.com>
Signed-off-by: Zach Brown <zach.brown@ni.com>

v2:
* If ubi_io_is_bad eraseblk_count_seq_show just returns the err.
* if ubi->lookuptbl returns null, its no longer treated as an error
  instead info for that block is not printeded
* Removed check for UBI_MAX_ERASECOUNTER since it is impossible to hit
* Removed block state from print, if a block is printed then it is good and
  if it is not printed, then it is bad.

v3:
* Remove errant ! symbol from if statement checking if erase count is valid.
Signed-off-by: Richard Weinberger <richard@nod.at>