riscv: Fix pmap_fault_fixup for L3 pages

riscv: Fix pmap_fault_fixup for L3 pages

Summary:
The parentheses being in the wrong place means that, for L3 pages,
oldpte has all bits except PTE_V cleared, and so all the subsequent
checks against oldpte will fail, causing us to bail out and not retry
the faulting instruction after an SFENCE.VMA. This causes a WITNESS +
INVARIANTS kernel to fault on the "Chisel P3" (BOOM-based) DARPA SSITH
GFE SoC in pmap_init when writing to pv_table and, being a nofault
entry, subsequently panic with:

  panic: vm_fault_lookup: fault on nofault entry, addr: 0xffffffc004e00000

Reviewed by:	markj
Approved by:	markj
Differential Revision:	https://reviews.freebsd.org/D24315