riscv: zero reserved PTE bits for L2 PTEs

As was done for L3 PTEs in r362853, mask out the reserved bits when
extracting the physical address from an L2 PTE. Future versions of the
spec or custom implementations may make use of these reserved bits, in
which case the resulting physical address could be incorrect.

Submitted by: Nathaniel Filardo <nwf20@cl.cam.ac.uk>
Reviewed by: kp, mhorne
Differential Revision: https://reviews.freebsd.org/D26607