Utilize pmap_enter(..., psind=1) in vm_fault_soft_fast() on amd64. (The
Differential Revision discusses the benefits of this change.)

Add a function, vm_reserv_to_superpage(), that returns the superpage
containing the specified base page.

Reviewed by: kib, markj
Tested by: pho
MFC after: 10 days
Differential Revision: https://reviews.freebsd.org/D11556