riscv: Fix pmap_kextract racing with concurrent superpage promotion/demotion
This repeats amd64's cfcbf8c6fd3b (r180498) and i386's cf3508519c5e
(r202894) but for riscv; pmap_kextract must be lock-free and so it can
race with superpage promotion and demotion, thus the L2 entry must only
be loaded once to avoid using inconsistent state.