buf_block_alloc(): ulint zip_size is always 0.
buf_LRU_get_free_block(): ulint zip_size is always 0.
buf_LRU_free_block(): ibool* buf_pool_mutex_released is always NULL.

Remove these parameters.

buf_LRU_get_free_block(): Simplify the initialization of block->page.zip
and release buf_pool_mutex() earlier.