truncate presently invalidates the dirty page's buffer_heads then shoots down
the page.  But try_to_free_buffers() will now bale out because the page is
dirty.

Net effect: the LRU gets filled with dirty pages which have invalidated
buffer_heads attached.  They have no ->mapping and hence cannot be cleaned.
The machine leaks memory at an enormous rate.

Fix this by cleaning the page before running try_to_free_buffers(), so
try_to_free_buffers() can do its work.

Also, remember to do dirty-page-acoounting in cancel_dirty_page() so the
machine won't wedge up trying to write non-existent dirty pages.

Probably still wrong, but now less so.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>