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
Change-Id: I80547a2aa944af2fb9ffde002650482877ade31b
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
(cherry picked from commit 7911fad5f0a2cf5afc2215657219a21e6630e001)