Revert the addition of VPO_BUSY and instead update vm_page_replace() to
properly unbusy the page.

Submitted by:	alc
Make sure vm_kmem_size is aligned on a page boundary, since that's what vmem
expects.
- The address lies in the bus space handle, not in the cookie
- Use the right address when calling kva_free()
Merge remote-tracking branch 'upstream/master'