Use pmap_enter(..., psind=1) in vm_fault_populate() on amd64. While
superpage mappings were already being created by automatic promotion in
vm_fault_populate(), this change reduces the cost of creating those
mappings. Essentially, one pmap_enter(..., psind=1) call takes the place
of 512 pmap_enter(..., psind=0) calls, and that one pmap_enter(...,
psind=1) call eliminates the allocation of a page table page.

Reviewed by: kib
MFC after: 10 days
Differential Revision: https://reviews.freebsd.org/D15572