nilfs2: separate read method of meta data files on super root block

Will displace nilfs_mdt_read_inode_direct function with an individual
read method: nilfs_dat_read, nilfs_sufile_read, nilfs_cpfile_read.

This provides the opportunity to initialize local variables of each
metadata file after reading the inode.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
diff --git a/fs/nilfs2/dat.h b/fs/nilfs2/dat.h
index 98f3306..d31c3aa 100644
--- a/fs/nilfs2/dat.h
+++ b/fs/nilfs2/dat.h
@@ -53,6 +53,7 @@
 int nilfs_dat_move(struct inode *, __u64, sector_t);
 ssize_t nilfs_dat_get_vinfo(struct inode *, void *, unsigned, size_t);
 
+int nilfs_dat_read(struct inode *dat, struct nilfs_inode *raw_inode);
 struct inode *nilfs_dat_new(struct the_nilfs *nilfs, size_t entry_size);
 
 #endif	/* _NILFS_DAT_H */