Fix race condition during cache attach

After attaching new cache device handle all the IOs in Pass-Through mode
until all the d2c requests are completed.

Signed-off-by: Robert Baldyga <robert.baldyga@huawei.com>

src/ocf_cache_priv.h

@@ -106,6 +106,8 @@ struct ocf_cache {
 	env_atomic flush_in_progress;
 	env_mutex flush_mutex;
 
+	env_atomic attach_pt;
+
 	struct ocf_cleaner cleaner;
 
 	struct list_head io_queues;