Write-only cache mode

Write-only cache mode is similar to writeback, however read
operations do not promote data to cache. Reads are mostly serviced
by the core device, only dirty sectors are fetched from the cache.

Signed-off-by: Adam Rutkowski <adam.j.rutkowski@intel.com>

inc/ocf_def.h

@@ -162,6 +162,9 @@ typedef enum {
 	ocf_cache_mode_wi,
 		/*!< Write invalidate cache mode */
 
+	ocf_cache_mode_wo,
+		/*!< Write-only cache mode */
+
 	ocf_cache_mode_max,
 		/*!< Stopper of cache mode enumerator */