arm64: Use pmap_early_vtophys in pmap_bootstrap_san

Use pmap_early_vtophys to find the physical address of the kernel base
rather than using the calculated offset as it will be removed in a
latter commit.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42566

(cherry picked from commit 5fae53589eacbf65855419922c881b3a25a68e03)