buf_buddy_alloc_zip() [UNIV_DEBUG]: Byte-fill the allocated blocks with ~i.

buf_buddy_block_free() [UNIV_DEBUG]: Clear the frame before releasing it
to the buffer pool.

buf_buddy_alloc_from() [UNIV_DEBUG]: Byte-fill the free blocks with j.

buf_buddy_relocate(): Allow the buf_page_hash_get() lookup to fail,
and note the reason.

buf_buddy_free_low(): Correctly compute the address of the combined free block.

buf_page_struct: Clarify the mutex protection rules.  Note that
the pointer zip.data is also protected by buf_pool->mutex.