nilfs2: do not allocate nilfs_mdt_info structure to gc-inodes

GC-inode now doesn't need the nilfs_mdt_info structure and there is no
reason that it is a sort of metadata files.

This stops the allocation and makes them not dependent on metadata
file routines.
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

fs/nilfs2/gcinode.c

@@ -168,7 +168,7 @@ int nilfs_gccache_wait_and_mark_dirty(struct buffer_head *bh)
 		}
 		nilfs_btnode_mark_dirty(bh);
 	} else {
-		nilfs_mdt_mark_buffer_dirty(bh);
+		nilfs_mark_buffer_dirty(bh);
 	}
 	return 0;
 }
@@ -177,11 +177,11 @@ int nilfs_init_gcinode(struct inode *inode)
 {
 	struct nilfs_inode_info *ii = NILFS_I(inode);
 	struct the_nilfs *nilfs = NILFS_SB(inode->i_sb)->s_nilfs;
-	int ret;
 
-	ret = nilfs_mdt_init(inode, nilfs, GFP_NOFS, 0);
-	if (!ret) {
+	inode->i_mode = S_IFREG;
+	mapping_set_gfp_mask(inode->i_mapping, GFP_NOFS);
 	inode->i_mapping->a_ops = &def_gcinode_aops;
+	inode->i_mapping->backing_dev_info = inode->i_sb->s_bdi;
 
 	ii->i_flags = 0;
 	nilfs_bmap_init_gc(ii->i_bmap);
@@ -193,8 +193,8 @@ int nilfs_init_gcinode(struct inode *inode)
 	 */
 	igrab(inode);
 	list_add(&NILFS_I(inode)->i_dirty, &nilfs->ns_gc_inodes);
-	}
-	return ret;
+
+	return 0;
 }
 
 /**