dm cache metadata: set dirty bits to clean on clean shutdown

When writing the dirty bitset to the metadata device on clean shutdown,
dirty bits were never being set to clean.  Harmless bug but this led to
confusion about whether there really was dirty data in the cache or not.

Reported-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>