Improve diagnostics.

buf_buddy_relocate(): Do not assume all small blocks to contain only one
buf_page_t object.  Require an exact size match, as in the case of
compressed pages.

buf_relocate(): Relocate buf_pool->LRU_old if needed.
Validate the LRU list in debug builds.

buf_LRU_block_remove_hashed_page(): Do not overwrite FIL_PAGE_OFFSET
or FIL_PAGE_ARCH_LOG_NO_OR_SPACE_ID.  The memory will be invalidated
in debug builds of buf_buddy_free().

buf0buddy.c: Add assertions for not holding buf_pool->zip_mutex.
The calling thread should not be holding any block->mutex, but it might
be too time-consuming to assert against that, even #ifdef UNIV_SYNC_DEBUG.