Simple code refactoring originally in D13484.

Extract swp_pager_force_dirty() and swp_pager_force_launder() out of
swp_pager_force_pagein().

Extract swap_pager_swapoff_object() out of swap_pager_swapoff().

Submitted by: ota_j.email.ne.jp
Reviewed by: alc, dougm
Approved by: kib (mentor)
Differential Revision: https://reviews.freebsd.org/D20545