the buffer pool mutex.  The instrumentation can be activated by
defining UNIV_DEBUG or UNIV_BUF_DEBUG at compilation time.

buf_pool_mutex_exit_forbidden: New variable.  When this is nonzero,
an assertion will fail in buf_pool_mutex_exit().

buf_pool_mutex_exit_forbid(): Macro for declaring that the buffer pool
mutex must not be released.  Calls may be nested.

buf_pool_mutex_exit_allow(): Macro for declaring that the buffer pool
mutex may be released.  Calls may be nested.

buf_LRU_search_and_free_block(): Prohibit buf_pool_mutex_exit() in the
scope of the function.

buf_LRU_free_block(): Prohibit buf_pool_mutex_exit() in buf_buddy_alloc()
and buf_buddy_free().

buf_LRU_block_remove_hashed_page(): Prohibit buf_pool_mutex_exit()
in buf_buddy_free().