riscv: Update a variable name to match a comment

This makes pmap_insert_pt_page() consistent with arm64 and amd64. No
functional change intended.

Reported by: alc
Fixes: 7703ac2e983b ("riscv: Port improvements from arm64/amd64 pmaps, part 1")

(cherry picked from commit 368b97361a0379acaec4a385c5c9d83ba6edc5a2)