ocfs2/trivial: Wrap ocfs2_sysfile_cluster_lock_key within define.

Actually ocfs2_sysfile_cluster_lock_key is only used if we enable
CONFIG_DEBUG_LOCK_ALLOC. Wrap it so that we can avoid a building
warning.
fs/ocfs2/sysfile.c:53: warning: ‘ocfs2_sysfile_cluster_lock_key’
defined but not used
Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>

fs/ocfs2/sysfile.c

@@ -50,7 +50,9 @@ static inline int is_in_system_inode_array(struct ocfs2_super *osb,
 					   int type,
 					   u32 slot);
 
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
 static struct lock_class_key ocfs2_sysfile_cluster_lock_key[NUM_SYSTEM_INODES];
+#endif
 
 static inline int is_global_system_inode(int type)
 {