nilfs_attach_log_writer(): use ->bd_mapping->host instead of ->bd_inode

I suspect that inode_attach_wb() use is rather unidiomatic, but
that's a separate story - in any case, its use is a few times
per mount *and* the route by which we access that inode is
"the host of address_space a page belongs to".

Signed-off-by: Al Viro <[email protected]>

Author: Al Viro

fs/nilfs2/segment.c

@@ -2790,7 +2790,7 @@ int nilfs_attach_log_writer(struct super_block *sb, struct nilfs_root *root)
 	if (!nilfs->ns_writer)
 		return -ENOMEM;
 
-	inode_attach_wb(nilfs->ns_bdev->bd_inode, NULL);
+	inode_attach_wb(nilfs->ns_bdev->bd_mapping->host, NULL);
 
 	err = nilfs_segctor_start_thread(nilfs->ns_writer);
 	if (unlikely(err))