nilfs2: add update functions of virtual block address to dat

This is a preparation for the successive cleanup ("nilfs2: allow btree
to directly call dat operations").

This adds functions bundling a few operations to change an entry of
virtual block address on the dat file.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
diff --git a/fs/nilfs2/dat.h b/fs/nilfs2/dat.h
index 91dc337..406070d 100644
--- a/fs/nilfs2/dat.h
+++ b/fs/nilfs2/dat.h
@@ -41,6 +41,12 @@
 int nilfs_dat_prepare_end(struct inode *, struct nilfs_palloc_req *);
 void nilfs_dat_commit_end(struct inode *, struct nilfs_palloc_req *, int);
 void nilfs_dat_abort_end(struct inode *, struct nilfs_palloc_req *);
+int nilfs_dat_prepare_update(struct inode *, struct nilfs_palloc_req *,
+			     struct nilfs_palloc_req *);
+void nilfs_dat_commit_update(struct inode *, struct nilfs_palloc_req *,
+			     struct nilfs_palloc_req *, int);
+void nilfs_dat_abort_update(struct inode *, struct nilfs_palloc_req *,
+			    struct nilfs_palloc_req *);
 
 int nilfs_dat_mark_dirty(struct inode *, __u64);
 int nilfs_dat_freev(struct inode *, __u64 *, size_t);