amd64 pmap_vm_page_alloc_check(): loose the assert

Current expression checks that vm_page_alloc(9) never returns a page
belonging to the preload area. This is not true if something was freed
from there, for instance a preloaded module was unloaded, or ucode update
freed.

Only check that we never allow to allocate a page belonging to the kernel
proper, check against _end.

Reported and tested by: dhw
Sponsored by: The FreeBSD Foundation
MFC after: 1 week