twice the maximum block size of the buddy system.

buf_page_t: Note that state may change from BUF_BLOCK_READY_FOR_USE
to BUF_BLOCK_MEMORY without buf_page_get_mutex() protection
[only buf_pool->mutex].

buf_buddy_get_slot(): Extend the output to BUF_BUDDY_SIZES.

buf_buddy_alloc(), buf_buddy_free(): Allow sizes up to UNIV_PAGE_SIZE.

buf_buddy_alloc_low(), buf_buddy_free_low(), buf_buddy_alloc_clean():
Allow i==BUF_BUDDY_SIZES.