ocfs2: Decrement refcount when truncating refcounted extents.

Add 'Decrement refcount for delete' in to the normal truncate
process. So for a refcounted extent record, call refcount rec
decrementation instead of cluster free.

Signed-off-by: Tao Ma <tao.ma@oracle.com>

fs/ocfs2/refcounttree.h

@@ -45,4 +45,10 @@ int ocfs2_decrease_refcount(struct inode *inode,
 			    handle_t *handle, u32 cpos, u32 len,
 			    struct ocfs2_alloc_context *meta_ac,
 			    struct ocfs2_cached_dealloc_ctxt *dealloc);
+int ocfs2_prepare_refcount_change_for_del(struct inode *inode,
+					  struct buffer_head *di_bh,
+					  u64 phys_blkno,
+					  u32 clusters,
+					  int *credits,
+					  struct ocfs2_alloc_context **meta_ac);
 #endif /* OCFS2_REFCOUNTTREE_H */