dm cache: fix writethrough mode quiescing in cache_map

Commit 2ee57d58 ("dm cache: add passthrough mode") inadvertently
removed the deferred set reference that was taken in cache_map()'s
writethrough mode support.  Restore taking this reference.

This issue was found with code inspection.
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>
Cc: stable@vger.kernel.org # 3.13+

drivers/md/dm-cache-target.c

@@ -2488,6 +2488,7 @@ static int cache_map(struct dm_target *ti, struct bio *bio)
 
 		} else {
 			inc_hit_counter(cache, bio);
+			pb->all_io_entry = dm_deferred_entry_inc(cache->all_io_ds);
 
 			if (bio_data_dir(bio) == WRITE && writethrough_mode(&cache->features) &&
 			    !is_dirty(cache, lookup_result.cblock))