Try reusing about-to-be-freed memory in swap_pager_reserve when desperate

[dougm]

Summary

If swp_pager_getswapspace fails in swap_pager_reserve, there may be enough swap space waiting to be freed to save the day, or at least to survive another iteration, so try that. That also prevents a possible small swap space leak when we run out of swap space while we're holding some waiting to be freed.