Add a new field to the md superblock, in an used area, to record where
resync was up-to on a clean shutdown while resync is active.  Restart from
this point.

The extra field is verified by having a second copy of the event counter.
If the second event counter is wrong, we ignore the extra field.

This patch thanks to  Angus Sawyer <angus.sawyer@dsl.pipex.com>