The following patch corrects a bug where rmap would continue trying to
swap out a page even after it failed on one pte, which could result in
leaked pte chains and a bug when exiting applications which use mlock().

The bug was tracked down by Christian Ehrhardt, the reason it wasn't
found earlier was a subtlety in the code, so I've taken the liberty of
changing Christian's patch into something more explicit, we shouldn't
let this one happen again ;)