buf_flush_freed_page(): Reformat in the common style, and
simplify some code. Prefer to request all information from
smaller data structures (buf_page_t) than from fil_space_t
or the global variable srv_immediate_scrub_data_uncompressed.

SysTablespace::open_or_create(): Assert that the temporary
tablespace will not be created in page_compressed format, so that
buf_flush_freed_page() can avoid checking that on every call.

IORequest: Remove duplicated constructors, and do not explicitly
declare a default constructor.