buf0flu.c: Allocate the temporary buffer from buf_frame_alloc().

page_zip_simple_validate(): Do not assert page_zip->m_start >= PAGE_DATA.

page_zip_compress(): Replace some assertions with page_zip_simple_validate(),
and do not assert anything about page_zip->data contents.

page_zip_validate(): Do not compare the page trailer bytes.

page_zip_write(): Assert that the entire page headers match and
that page_zip->m_start >= PAGE_DATA.