vm_page: Allow PG_NOFREE pages to be freed

There is at least one case where we need to support it: kmem_malloc()
might need to allocate multiple pages to satisfy a NOFREE allocation,
which it implements by calling vm_page_alloc() in a loop. If it fails
part-way though, it needs to free already-allocated pages, but this was
illegal.

Convert the bump allocator to a linked list; (ab)use the pindex field of
each page in the list to store the number of contiguous pages in the
block. (Originally I added a new plinks member for this purpose, but
it's not safe to use that until after vm_page_dequeue() is called due to
lazy page queue removal.) Then, modify vm_page_free() to support freeing
pages to this list.

While here, add a __noinline qualifier to vm_page_alloc_nofree_domain()
to ensure that it doesn't get inlined into a hot path.

Reported by: syzbot+93bc9edd2d0f22ae426a@syzkaller.appspotmail.com
Reviewed by: bnovkov, kib
Fixes: a8693e89e3e4 ("vm: Introduce vm_page_alloc_nofree_domain")
Differential Revision: https://reviews.freebsd.org/D49480