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.