dm cache: log error message if dm_kcopyd_copy() fails

A migration failure should be logged (albeit limited).

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
index dc63eb2..05a10c0 100644
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -886,8 +886,10 @@
 		r = dm_kcopyd_copy(cache->copier, &o_region, 1, &c_region, 0, copy_complete, mg);
 	}
 
-	if (r < 0)
+	if (r < 0) {
+		DMERR_LIMIT("issuing migration failed");
 		migration_failure(mg);
+	}
 }
 
 static void avoid_copy(struct dm_cache_migration *mg)