dm cache: set/clear the cache core's dirty_bitset when loading mappings

When loading metadata make sure to set/clear the dirty bits in the cache
core's dirty_bitset as well as the policy.

Otherwise the cache core is unaware that any blocks were dirty when the
cache was last shutdown.  A very serious side-effect being that the
cleaner policy would therefore never be tasked with writing back dirty
data from a cache that was in writeback mode (e.g. when switching from
smq policy to cleaner policy when decommissioning a writeback cache).

This fixes a serious data corruption bug associated with writeback mode.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-cache-target.c

@@ -2960,6 +2960,12 @@ static int load_mapping(void *context, dm_oblock_t oblock, dm_cblock_t cblock,
 	int r;
 	struct cache *cache = context;
 
+	if (dirty) {
+		set_bit(from_cblock(cblock), cache->dirty_bitset);
+		atomic_inc(&cache->nr_dirty);
+	} else
+		clear_bit(from_cblock(cblock), cache->dirty_bitset);
+
 	r = policy_load_mapping(cache->policy, oblock, cblock, dirty, hint, hint_valid);
 	if (r)
 		return r;