btrfs: switch extent_io_tree::private_data to btrfs_inode and rename

The extent_io_tree::private_data was meant to be a preparatory work for
the metadata inode rework but that never materialized. Now it's used
only for an inode so it's better to change the appropriate type and
rename it.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>