Based on work posted at https://reviews.freebsd.org/D13484, change
swap_pager_swapoff_object and swp_pager_force_pagein so that they can
page in multiple pages at a time to a swap device, rather than doing
one I/O operation for each page.

Tested by: pho
Submitted by: ota_j.email.ne.jp (Yoshihiro Ota)
Reviewed by: alc, markj, kib
Approved by: kib, markj (mentors)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D20635