swap_pager: add new page range struct

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.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D45562