simple_prepare_write() currently memsets the entire page.  It only
needs to clear the parts which are outside the to-be-written region.
This change makes no difference to performance - that memset was just a
cache preload for the copy_from_user() in generic_file_write().  But
it's more correct.

Also, mark the page dirty in simple_commit_write(), not in
simple_prepare_write().  Because the page's contents are changed after
prepare_write().  This doesn't matter in practice, but it is setting a
bad example.

Also, add a flush_dcache_page() to simple_prepare_write().  Again, not
really needed because the page cannot be mapped into pagetables if it
is not uptodate.  But it is example code and should not be missing such
things.