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.

Approved by: kib (mentor)
Differential Revision: https://reviews.freebsd.org/D20228