Split global metadata lock

Divide single global lock instance into 4 to reduce contention
in multiple read-locks scenario.

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

src/ocf_request.h

@@ -9,6 +9,7 @@
 #include "ocf_env.h"
 #include "ocf_io_priv.h"
 #include "engine/cache_engine.h"
+#include "metadata/metadata_structs.h"
 
 struct ocf_req_allocator;
 
@@ -190,6 +191,9 @@ struct ocf_request {
 	uint8_t part_evict : 1;
 	/* !< Some cachelines from request's partition must be evicted */
 
+	uint8_t lock_idx : OCF_METADATA_GLOBAL_LOCK_IDX_BITS;
+	/* !< Selected global metadata read lock */
+
 	log_sid_t sid;
 	/*!< Tracing sequence ID */