mdcheck: Log when done

mdcheck: Log when done

Currently mdcheck (when called with `--duration`) logs only the
beginning of the check, the pausing and the continuation but not the
completion.

So, log the completion, too, so that it can be determined how long the
raid check took.

    2020-05-08T18:00:02+02:00 deadpool root: mdcheck start checking /dev/md0
    2020-05-08T18:00:02+02:00 deadpool root: mdcheck start checking /dev/md1
    2020-05-09T15:32:04+02:00 deadpool root: mdcheck finished checking /dev/md1
    2020-05-09T17:38:04+02:00 deadpool root: mdcheck finished checking /dev/md0

Cc: linux-raid@vger.kernel.org
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>