This fixes a bug in ext3 - when ext3 decides that it wants to fail its
writepage(), it is running SetPageDirty().  But ->writepage has just put
the page on ->clean_pages().  The page ends up dirty, on ->clean_pages
and the normal writeback paths don't know about it any more.

So run set_page_dirty() instead, to place the page back on the dirty
list.

And in move_from_swap_cache(), shuffle the page across to ->dirty_pages
so that it's eligible for writeout.  ___add_to_page_cache() forgets to
look at the page state when deciding which list to attach it to.

All SetPageDirty() callers otherwise look OK.