commit ad6171b9 (MDEV-22456)
introduced code to buf_page_create() that would lazily drop
adaptive hash index entries for an index that has been
evicted from the data dictionary cache.

Unfortunately, that call was missing adequate protection.
While the btr_search_drop_page_hash_index(block) was executing,
the block could be reused for something else.

buf_page_create(): If btr_search_drop_page_hash_index() must be
invoked, pin the block before releasing the buf_pool->page_hash lock,
so that the block cannot be grabbed by other threads.