Introduce vm_phys_enq_range(), and call it in vm_phys_alloc_npages()
and vm_phys_alloc_seg_contig() instead of vm_phys_free_contig(). In
short, vm_phys_enq_range() is simpler and faster than the more general
vm_phys_free_contig(), and in the case of vm_phys_alloc_seg_contig(),
vm_phys_free_contig() was placing the excess physical pages at the
wrong end of the queues.

In collaboration with: Doug Moore <dougm@rice.edu>