ANDROID: dm verity fec: add sysfs attribute fec/corrected

Add a sysfs entry that allows user space to determine whether dm-verity
has come across correctable errors on the underlying block device.

Bug: 22655252
Bug: 27928374
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
(cherry picked from commit 7911fad5f0a2cf5afc2215657219a21e6630e001)

Change-Id: I02d36bed1d54eaf1f1750a919fa91f66a7e9b96c
Git-commit: 9cd83628
Git-repo: https://git.linaro.org/kernel/linux-linaro-stable.git


Signed-off-by: Imran Khan <kimran@codeaurora.org>