riscv: fix a bug in calculating the pindex for L1 page

pmap_l1_pindex(va) expands to: ((va >> L1_SHIFT) + NUL2E)

Reviewed by: mhorne
MFC after: 1 week
Fixes: a4667e09e652 ("Convert vm_page_alloc() callers to...")
Pull Request: https://github.com/freebsd/freebsd-src/pull/1360