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_queue_priv.h

@@ -18,6 +18,9 @@ struct ocf_queue {
 	struct list_head io_list;
 	env_spinlock io_list_lock;
 
+	/* per-queue free running global metadata lock index */
+	unsigned lock_idx;
+
 	/* Tracing reference counter */
 	env_atomic64 trace_ref_cntr;