buf_flush_page(): Never wait for a page latch, even in checkpoint
flushing (flush_type == BUF_FLUSH_LIST), to prevent a hang of the
page cleaner threads when a large number of pages is latched.

In mysql/mysql-server@9542f3015b00330ef537f6223565b28b82a5b325
it was claimed that such a hang only affects CREATE FULLTEXT INDEX.
Their fix was to retain buffer-fix but release exclusive latch
on non-leaf pages, and subsequently write to those pages while
they are not associated with the mini-transaction, which would
trip a debug assertion in the MariaDB version of
mtr_t::memo_modify_page() and cause potential corruption
when using the default MariaDB setting innodb_log_optimize_ddl=OFF.

This change essentially backports a small part of
commit 7cffb5f6 (MDEV-23399)
from MariaDB Server 10.5.7.