buf_page_get_gen(): Once again, zip_size must always match the
compressed page size of the tablespace where the page is requested from.
This seems to hold also for all calls from the insert buffer.

buf_page_optimistic_get_func(): Do not pass mtr to ibuf_page(), because
there is no guarantee that mtr would contain an x-latch to the insert
buffer bitmap page that covers (block->page.space, block->page.offset).