More new code for 5.13: - Rename the log timestamp struct. - Remove broken transaction counter debugging that wasn't working correctly on very old filesystems. - Various fixes to make pre-lazysbcount filesystems work properly again. - Fix a free space accounting problem where we neglected to consider free space btree blocks that track metadata reservation space when deciding whether or not to allow caller to reserve space for a metadata update. - Fix incorrect pagecache clearing behavior during FUNSHARE ops. - Don't allow log writes if the data device is readonly.