There is something wrong with the management of compressed-only blocks
(BUF_BLOCK_ZIP_PAGE).  To disable the creation of such blocks, set zip=TRUE
in buf_LRU_block_remove_hashed_page().

buf_LRU_block_remove_hashed_page(): Release buf_pool->zip_mutex when
freeing a compressed-only page and its control block, with buf_buddy_free().
Adapt callers.

buf_LRU_block_free_hashed_page(): Change the parameter type from buf_page_t*
to buf_block_t*.

buf_LRU_free_block(): Move below the definition of buf_LRU_add_block_low().
Allocate block descriptor for compressed-only blocks.

buf_LRU_block_free_non_file_page(): Replace ut_free() with buf_buddy_free().

buf_zip_decompress(): New function, split from buf_page_io_complete().

buf_page_init_for_read(): Do not allocate the compressed page until it is
really needed, to avoid the overhead of the buddy allocator.  Decompress
compressed-only blocks that are needed again.

buf_page_create(): Do not allocate the compressed page until it is
really needed, to avoid the overhead of the buddy allocator.

buf_validate(): Replace some equality tests on the lengths of the LRU
list and the flush lists with greater-or-equal tests, since the counted
numbers do not include control blocks for compressed-only pages.