Starting with commit 1a6f708e
the function buf_pool_get_dirty_pages_count() is only used
in a debug check. It was dead code for non-debug builds.

buf_flush_dirty_pages(): Perform the debug check inline,
and replace the assertion
	ut_ad(first || buf_pool_get_dirty_pages_count(id) == 0);
with another one that is executed while holding the mutexes:
	ut_ad(id != bpage->id.space());