Introduce d2c request

This avoids unnecessary map allocation and initialization of unused fields of
request structure. It also allows to track thier number separately from
the regular requests

Signed-off-by: Robert Baldyga <robert.baldyga@huawei.com>
Signed-off-by: Michal Mielewczyk <michal.mielewczyk@huawei.com>

src/mngt/ocf_mngt_cache.c

@@ -857,6 +857,8 @@ static int _ocf_mngt_init_new_cache(struct ocf_cache_mngt_init_params *params)
 	/* start with freezed metadata ref counter to indicate detached device*/
 	ocf_refcnt_freeze(&cache->refcnt.metadata);
 
+	ocf_refcnt_init(&cache->refcnt.d2c);
+
 	env_atomic_set(&(cache->last_access_ms),
 			env_ticks_to_msecs(env_get_tick_count()));