Improve swp_pager_force_pagein() such that "swapoff" reads multiple blocks at a time.
I used vm_thread_swapin() as a reference on how to grab pages.
Call swap_pager_haspages() to find out extra and continuous VM pages and then call vm_page_grab() to prepare multiple vm_pages_t.
swap_pager_getpages() calls vm_page_alloc() for rahead but it looks we need to call vm_page_grab(s) instead for page-in purpose.
We find up to SWB_NPAGES (32 for i386) by swap_pager_haspages().
While SWAP_META_PAGES (8 for i386) is less than SWAP_META_PAGES, we skip "sb" blocks accordingly.
It looks VM_PAGE_BITS_ALL is set for pages that have been put back from swap devices. I find regions of non-VM_PAGE_BITS_ALL for npages to getpages back.