The function is identical in each minidump implementation, and it is
small, so move it to the vm_dumpset.h header. The only slight exception
is powerpc where the function is public, for use in moea64_scan_pmap().
This is a good cleanup. I don't really like the header pollution. We are already assuming that consumers have included vm_page.h (to check m->flags & PG_NODUMP), so why not vm_phys.h as well? Alternately we could have the implementation live in vm_phys.c and call it vm_phys_is_dumpable() or so.