It can useful for code outside the VM system to look up the NUMA domain
of a page backing a virtual or physical address, specifically when
creating NUMA-aware data structures.  We have _vm_phys_domain() for
this, but the leading underscore implies that it's an internal function,
and vm_phys.h has dependencies on a number of other headers.

Rename vm_phys_domain() to vm_page_domain(), and _vm_phys_domain() to
vm_phys_domain().  Make the latter an inline function.

Add _vm_phys.h and define struct vm_phys_seg there so that it's easier
to use in other headers.  Include it from vm_page.h so that
vm_page_domain() can be defined there.

Include machine/vmparam.h from _vm_phys.h since it depends directly on
some constants defined there.

Reviewed by:	alc
Reviewed by:	dougm, kib (earlier versions)
Differential Revision:	https://reviews.freebsd.org/D27207