In the rewrite of MDEV-8139 (based on MDEV-15528), we introduced a
wrong assumption that any persistent tablespace that is not an .ibd
file is the system tablespace. This assumption is broken when
innodb_undo_tablespaces (files undo001, undo002, ...) are being used.
By default, we have innodb_undo_tablespaces=0 (the persistent undo
log is being stored in the system tablespace).

In MDEV-15528 and MDEV-8139 we rewrote the page scrubbing logic
so that it will follow the tried-and-true write-ahead logging
protocol, first writing FREE_PAGE records and then in the page
flushing, zerofilling or hole-punching freed pages.

Unfortunately, the implementation included a wrong assumption that
that anything that is not in an .ibd file must be the system tablespace.
This wrong assumption would cause overwrites of valid data pages in
the system tablespace.

mtr_t::m_freed_in_system_tablespace: Remove.

mtr_t::m_freed_space: The tablespace associated with m_freed_pages.

buf_page_free(): Take the tablespace and page number as a parameter,
instead of taking a page identifier.