MDEV-25801: buf_flush_dirty_pages() is very slow

In commit 7cffb5f (MDEV-23399)
the implementation of buf_flush_dirty_pages() was replaced with
a slow one, which would perform excessive scans of the
buf_pool.flush_list and make little progress.

buf_flush_list(), buf_flush_LRU(): Split from buf_flush_lists().
Vladislav Vaintroub noticed that we will not need to invoke
log_flush_task.wait() for the LRU eviction flushing.

buf_flush_list_space(): Replaces buf_flush_dirty_pages().
This is like buf_flush_list(), but operating on a single
tablespace at a time. Writes at most innodb_io_capacity
pages. Returns whether some of the tablespace might remain
in the buffer pool.

storage/innobase/buf/buf0buf.cc

@@ -1729,10 +1729,10 @@ inline bool buf_pool_t::withdraw_blocks()
 
 		/* reserve free_list length */
 		if (UT_LIST_GET_LEN(withdraw) < withdraw_target) {
-			ulint n_flushed = buf_flush_lists(
+			ulint n_flushed = buf_flush_LRU(
 				std::max<ulint>(withdraw_target
 						- UT_LIST_GET_LEN(withdraw),
-						srv_LRU_scan_depth), 0);
+						srv_LRU_scan_depth));
 			buf_flush_wait_batch_end_acquiring_mutex(true);
 
 			if (n_flushed) {
@@ -3321,7 +3321,7 @@ buf_page_get_low(
 
 		fix_block->fix();
 		mysql_mutex_unlock(&buf_pool.mutex);
-		buf_flush_lists(ULINT_UNDEFINED, LSN_MAX);
+		buf_flush_list();
 		buf_flush_wait_batch_end_acquiring_mutex(false);
 
 		if (fix_block->page.buf_fix_count() == 1