ubi: Implement ioctl for detailed erase counters

Currently, "max_ec" can be read from sysfs, which provides a limited
view of the flash device’s wear. In certain cases, such as bugs in
the wear-leveling algorithm, specific blocks can be worn down more
than others, resulting in uneven wear distribution. Also some use cases
can wear the erase blocks of the fastmap area more heavily than other
parts of flash.
Providing detailed erase counter values give a better understanding of
the overall flash wear and is needed to be able to calculate for example
expected life time.
There exists more detailed info in debugfs, but this information is
only available for debug builds.

Signed-off-by: Rickard Andersson <rickard.andersson@axis.com>
Tested-by: Zhihao Cheng <chengzhihao1@huawei.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>