In FreeBSD 7.0 and beyond, pmap_growkernel() should pass VM_ALLOC_INTERRUPT
to vm_page_alloc() instead of VM_ALLOC_SYSTEM. VM_ALLOC_SYSTEM was the
logical choice before FreeBSD 7.0 because VM_ALLOC_INTERRUPT could not
reclaim a cached page. Simply put, there was no ordering between
VM_ALLOC_INTERRUPT and VM_ALLOC_SYSTEM as to which "dug deeper" into the
cache and free queues. Now, there is; VM_ALLOC_INTERRUPT dominates
VM_ALLOC_SYSTEM.

While I'm here, teach pmap_growkernel() to request a prezeroed page.

MFC after: 1 week