nilfs2: fix lockdep recursive locking warning on meta data files

This fixes the following false detection of lockdep against nilfs meta
data files:

=============================================
[ INFO: possible recursive locking detected ]
2.6.29 #26
---------------------------------------------
mount.nilfs2/4185 is trying to acquire lock:
 (&mi->mi_sem){----}, at: [<d0c7925b>] nilfs_sufile_get_stat+0x1e/0x105 [nilfs2]
 but task is already holding lock:
  (&mi->mi_sem){----}, at: [<d0c72026>] nilfs_count_free_blocks+0x48/0x84 [nilfs2]
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

fs/nilfs2/the_nilfs.c

@@ -115,6 +115,7 @@ void put_nilfs(struct the_nilfs *nilfs)
 static int nilfs_load_super_root(struct the_nilfs *nilfs,
 				 struct nilfs_sb_info *sbi, sector_t sr_block)
 {
+	static struct lock_class_key dat_lock_key;
 	struct buffer_head *bh_sr;
 	struct nilfs_super_root *raw_sr;
 	struct nilfs_super_block **sbp = nilfs->ns_sbp;
@@ -163,6 +164,9 @@ static int nilfs_load_super_root(struct the_nilfs *nilfs,
 	if (unlikely(err))
 		goto failed_sufile;
 
+	lockdep_set_class(&NILFS_MDT(nilfs->ns_dat)->mi_sem, &dat_lock_key);
+	lockdep_set_class(&NILFS_MDT(nilfs->ns_gc_dat)->mi_sem, &dat_lock_key);
+
 	nilfs_mdt_set_shadow(nilfs->ns_dat, nilfs->ns_gc_dat);
 	nilfs_mdt_set_entry_size(nilfs->ns_cpfile, checkpoint_size,
 				 sizeof(struct nilfs_cpfile_header));