MDEV-27461: Buffer pool resize fails to wake up the page cleaner

buf_pool_t::realloc(): Invoke page_cleaner_wakeup()
if buf_LRU_get_free_only() returns a null pointer.

Ever since commit 7b1252c0 (MDEV-24278)
the page cleaner would remain in untimed sleep, expecting explicit
calls to buf_pool_t::page_cleaner_wakeup() when the ratio of dirty pages
could change.

Failure to wake up the page cleaner will cause all page writes to be
initiated by buf_flush_LRU_list_batch(). That might work too,
provided that the buffer pool size is at least BUF_LRU_MIN_LEN (256)
pages, but it would not advance the log checkpoint.

storage/innobase/buf/buf0buf.cc

@@ -1571,6 +1571,9 @@ inline bool buf_pool_t::realloc(buf_block_t *block)
 	new_block = buf_LRU_get_free_only();
 
 	if (new_block == NULL) {
+		mysql_mutex_lock(&buf_pool.flush_list_mutex);
+		page_cleaner_wakeup();
+		mysql_mutex_unlock(&buf_pool.flush_list_mutex);
 		return(false); /* free list was not enough */
 	}