buf_page_free(): Flag the freed page as modified if it is found in
the buffer pool.

buf_flush_page(): If the page has been freed, ensure that the log
for it has been durably written, before removing the page
from buf_pool.flush_list.

FindBlockX: Find also MTR_MEMO_PAGE_X_MODIFY in order to avoid an
occasional failure of innodb.innodb_defrag_concurrent, which involves
freeing and reallocating pages in the same mini-transaction.

This fixes a regression that was introduced in
commit a35b4ae8 (MDEV-15528).

This logic was tested by commenting out the $shutdown_timeout line
from a test and running the following:

./mtr --rr innodb.scrub
rr replay var/log/mysqld.1.rr/mariadbd-0

A breakpoint in the modified buf_flush_page() was hit, and the
FIL_PAGE_LSN of that page had been last modified during the
mtr_t::commit() of a mini-transaction where buf_page_free()
had been executed on that page.