fs: btrfs: Use LZO_LEN to replace immediate number

Just a cleanup. These immediate numbers make my eyes hurt.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Cc: Marek Behun <marek.behun@nic.cz>
Reviewed-by: Marek Behún <marek.behun@nic.cz>