Fixes a bug in generic_writepages() and its cut-n-paste-cousin,
mpage_writepages().

The code was clearing PageDirty and then baling out if it discovered
the page was nder writeback.  Which would cause the dirty bit to be
lost.

It's a very small window, but reversing the order so PageDirty is only
cleared when we know for-sure that IO will be started fixes it up.