log_sort_flush_list(): Wait for any pending page writes to cease
before sorting the buf_pool.flush_list. Starting with
commit 22b62eda (MDEV-25113),
it is possible that some buf_page_t::oldest_modification_
that we will be comparing in std::sort() will be updated from
some value >2 to 1 while we are holding buf_pool.flush_list_mutex.

To catch this type of trouble better in the future, we will clean
garbage (pages that have been written out) from buf_pool.flush_list
while constructing the array for sorting, and check with debug
assertions that all blocks that we are copying from the array to the
list will be dirty (requiring a writeback) while we sort and copy
the array back to buf_pool.flush_list.

This failure was observed by chance exactly once when running the test
innodb.recovery_memory. It was never reproduced in the same form
afterwards. Unrelated to this change, the test does occasionally
reproduce a failure to start up InnoDB due to a corrupted page
being read in recovery. The ticket MDEV-31791 was filed for that.

Tested by: Matthias Leich