swap_pager: add new page range struct

[dougm]

Summary

Define a page_range struct to pair up the two values passed to
freerange functions. Have swp_pager_freeswapspace also take a
page_range argument rather than a pair of arguments.

In swp_pager_meta_free_all, drop a needless test and use a new
helper function to do the cleanup for each swap block.

I expect the new helper function to be essential for a future change
to swp_pager_meta_free_all, as a callback function.

Test Plan

I've built and booted a kernel with this change.