In commit 0d175968 (MDEV-31354)
we only waited that no buf_pool.flush_list writes are in progress.
The buf_flush_page_cleaner() thread could still initiate page writes
from the buf_pool.LRU list while only holding buf_pool.mutex, not
buf_pool.flush_list_mutex. This is something that was changed in
commit a55b951e (MDEV-26827).

log_sort_flush_list(): Wait for the buf_flush_page_cleaner() thread to
be completely idle, including LRU flushing.

buf_flush_page_cleaner(): Always broadcast buf_pool.done_flush_list
when becoming idle, so that log_sort_flush_list() will be woken up.
Also, ensure that buf_pool.n_flush_inc() or
buf_pool.flush_list_set_active() has been invoked before any page
writes are initiated.

buf_flush_try_neighbors(): Release buf_pool.mutex here and not in the
callers, to avoid code duplication. Make innodb_flush_neighbors=ON
obey the innodb_io_capacity limit.