trx_purge_truncate_history(): Do not prematurely mark dirty pages
as clean. This will be done in mtr_t::commit_shrink() as part of
Shrink::operator()(mtr_memo_slot_t*). Also, register each dirty page
only once in the mini-transaction.

fsp_page_create(): Adjust and simplify the page creation during
undo tablespace truncation. We can directly reuse pages that are
already in buf_pool.page_hash.

This fixes a regression that was caused by
commit f5794e1d (MDEV-26445).

Tested by: Matthias Leich
Reviewed by: Thirunarayanan Balathandayuthapani