I've had this patch hanging around for a couple of months (you liked an
earlier version, but I never found time to resubmit it), remove some
unnecessary PageDirty and PageUptodate manipulations.

add_to_page_cache can only receive a dirty page in the add_to_swap
case, so deal with it there.  add_to_swap is better off using
add_to_page_cache directly than add_to_swap_cache.  Keep move_to_ and
_from_swap_cache simple, and don't fiddle with flags without reason.
It's a little less efficient to correct clean->dirty list as an
afterthought, but cuts unusual code from slow path.