xfs: implement deferred bmbt map/unmap operations

Implement deferred versions of the inode block map/unmap functions.
These will be used in subsequent patches to make reflink operations
atomic.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
index 460ea79..61b7fbd 100644
--- a/fs/xfs/xfs_trans.h
+++ b/fs/xfs/xfs_trans.h
@@ -40,6 +40,7 @@
 struct xfs_cud_log_item;
 struct xfs_defer_ops;
 struct xfs_bui_log_item;
+struct xfs_bud_log_item;
 
 typedef struct xfs_log_item {
 	struct list_head		li_ail;		/* AIL pointers */