Running mysqld with innodb-buffer-pool-instances > 1 hangs on startup.

On startup wrong variables was being used to detect number of page cleaner
threads. As a result no threads were actually started. And subsequent code
waits for threads to start forever.

Fixed by using page_cleaner->n_workers, which holds number of page cleaner
threads (0 at startup) instead of srv_n_page_cleaners, which holds number
of requested page cleaner threads (4 by default).