vm_page: Implement lazy page initialization

FreeBSD's boot times have decreased to the point where vm_page array
initialization represents a significant fraction of the total boot time.
For example, when booting FreeBSD in Firecracker (a VMM designed to
support lightweight VMs) with 128MB and 1GB of RAM, vm_page
initialization consumes 9% (3ms) and 37% (21.5ms) of the kernel boot
time, respectively. This is generally relevant in cloud environments,
where one wants to be able to spin up VMs as quickly as possible.

This patch implements lazy initialization of (most) page structures,
following a suggestion from cperciva@. The idea is to introduce a new
free pool, VM_FREEPOOL_LAZYINIT, into which all vm_page structures are
initially placed. For this to work, we need only initialize the first
free page of each chunk placed into the buddy allocator. Then, early
page allocations draw from the lazy init pool and initialize vm_page
chunks (up to 16MB, 4096 pages) on demand. Once APs are started, an
idle-priority thread drains the lazy init pool in the background to
avoid introducing extra latency in the allocator. With this scheme,
almost all of the initialization work is moved out of the critical path.

A couple of vm_phys operations require the pool to be drained before
they can run: vm_phys_find_range() and vm_phys_unfree_page(). However,
these are rare operations. I believe that
vm_phys_find_freelist_contig() does not require any special treatment,
as it only ever accesses the first page in a power-of-2-sized free page
chunk, which is always initialized.

For now the new pool is only used on amd64 and arm64, since that's where
I can easily test and those platforms would get the most benefit.

Reviewed by: alc, kib
Differential Revision: https://reviews.freebsd.org/D40403