In vm_page_startup(), we were performing two vm_phys_segs searches per
page to look up information that we already have. I don't think it's an
abstraction violation to avoid the lookups given how much
vm_page_startup() already knows about the phys segs array.

Garbage-collect the now-unused vm_phys_paddr_to_segind() function.

This reduced the number of cycles spent initializing vm_page_array by
13% on a desktop Ivy Bridge system that I tested.