cache: improve activation with -real

When cache volume may be converted from normal to -real layer LV
we need to improve logic for call cache_check.

With this patch, we register call for cache_check only when metadata LV
is not yet present in active table slot (should match initial table
load).
This avoids unwanted checking when cache would become layer device
online.

WHATS_NEW

@@ -1,5 +1,6 @@
 Version 2.02.169 - 
 =====================================
+  Support cache volume activation with -real layer.
   Improve search of lock-holder for external origin and thin-pool.
   Support status checking of cache volume used in layer.
   Avoid shifting by one number of blocks when clearing dirty cache volume.

lib/activate/dev_manager.c

@@ -2901,6 +2901,10 @@ static int _add_new_lv_to_dtree(struct dev_manager *dm, struct dm_tree *dtree,
 		return_0;
 
 	if (lv_is_cache(lv) &&
+	    /* Register callback only for layer activation or non-layered cache LV */
+	    (layer || !lv_layer(lv)) &&
+	    /* Register callback when metadata LV is NOT already active */
+	    !_cached_dm_info(dm->mem, dtree, first_seg(first_seg(lv)->pool_lv)->metadata_lv, NULL) &&
 	    !_pool_register_callback(dm, dnode, lv))
 		return_0;