amd64 pmap: Optimize PKU lookups when creating superpage mappings

Modify pmap_pkru_same() to update the prototype PTE at the same time as
checking the address range. This eliminates the need for calling
pmap_pkru_get() in addition to pmap_pkru_same(). pmap_pkru_same() was
already doing most of the work of pmap_pkru_get().

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D46135