In commit 3a9a3be1 (MDEV-23855)
some previous logic was replaced with the condition
dirty_pct < srv_max_dirty_pages_pct_lwm, which caused
the default value of the parameter innodb_max_dirty_pages_pct_lwm=0
to lose its special meaning: 'refer to innodb_max_dirty_pages_pct instead'.

This implicit special meaning was visible in the function
af_get_pct_for_dirty(), which was removed in
commit f0c295e2 (MDEV-24369).

page_cleaner_flush_pages_recommendation(): Restore the special
meaning that was removed in MDEV-24369.

buf_flush_page_cleaner(): If srv_max_dirty_pages_pct_lwm==0.0,
refer to srv_max_buf_pool_modified_pct. This fixes the observed
performance regression due to excessive page flushing.

buf_pool_t::page_cleaner_wakeup(): Revise the wakeup condition.

innodb_init(): Do initialize srv_max_io_capacity in Mariabackup.
It was previously constantly 0, which caused mariadb-backup --prepare
to hang in buf_flush_sync(), making no progress.