dm cache: metadata clear dirty bits on clean shutdown

When writing the dirty bitset to the metadata device on a clean
shutdown, clear the dirty bits.  Previously they were left indicating
the cache was dirty. This led to confusion about whether there really
was dirty data in the cache or not.  (This was a harmless bug.)
Reported-by: NDarrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: NJoe Thornber <ejt@redhat.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>

drivers/md/dm-cache-metadata.c

@@ -979,7 +979,7 @@ static int __dirty(struct dm_cache_metadata *cmd, dm_cblock_t cblock, bool dirty
 		/* nothing to be done */
 		return 0;
 
-	value = pack_value(oblock, flags | (dirty ? M_DIRTY : 0));
+	value = pack_value(oblock, (flags & ~M_DIRTY) | (dirty ? M_DIRTY : 0));
 	__dm_bless_for_disk(&value);
 
 	r = dm_array_set_value(&cmd->info, cmd->root, from_cblock(cblock),