Let swp_pager_getswapspace manage request resizing

[dougm]

Summary

Callers of swp_pager_getswapspace get either as many blocks as they requested, or none, and in the latter case it is up to them to pick a smaller request to make - which they always do by halving the failed request. This change to swp_pager_getswapspace leaves the task of downsizing the request to the function and not its caller. It still does so by halving the original request.

[dougm]

Drop an excess mpages reference.