UBI: add empty eraseblocks verification

This patch adds code which makes sure eraseblocks contain all 0xFF
bytes before starting using them. The verification is done only when
debugging checks are enabled.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

diff --cc drivers/mtd/ubi/debug.h
Simple merge

diff --cc drivers/mtd/ubi/io.c
Simple merge

diff --cc drivers/mtd/ubi/wl.c
Simple merge