[RFC] UBI statistics and bitrot interface

[Richard Weinberger]

This series adds new ioctl()s to UBI such that userspace can query
read/erase counters and trigger re-reads/scrubbing of PEBs.
Especially for MLC NAND flash where read disturb and data retention
is a big topic this interface can be used by applications to foster the NAND.
Parallel to this series at patch series against mtd-utils is sent that
implements a ubihealthd which makes use of that interface.

[PATCH 1/5] UBI: Introduce prepare_erase_work()
[PATCH 2/5] UBI: Introduce in_pq()
[PATCH 3/5] UBI: Expose the bitrot interface
[PATCH 4/5] UBI: Add basic read counter support
[PATCH 5/5] UBI: Expose UBI statistics interface