Fix a set but not used warning in the arm64 pmap

In pmap_ts_referenced we read the virtual address from pv->pv_va,
but then continue to use the pv struct to get the same value later
in the function.

Use the virtual address value we initially read rather than loading
from the pv struct each time.

(cherry picked from commit 1c643b721bed48ba795b42cdc5e8b5818f30ed14)