riscv: fix vm.pmap.kernel_maps with Sv48

With 4-level paging enabled, the layout of KVA is identical, but we need
to step through an extra level to find the L1 table.

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45473