Since commit bd5a6403 (MDEV-26033)
we can actually calculate the buf_pool.page_hash cell and latch
addresses while not holding buf_pool.mutex.

buf_page_alloc_descriptor(): Remove the MEM_UNDEFINED.
We now expect buf_page_t::hash to be zero-initialized.

buf_pool_t::hash_chain: Dedicated data type for buf_pool.page_hash.array.

buf_LRU_free_one_page(): Merged to the only caller
buf_pool_t::corrupted_evict().