RISC-V: Fix a race condition during kernel stack overflow

RISC-V: Fix a race condition during kernel stack overflow

This fixes a concrete bug but is also the basis for some cleanup work,
so I'm merging it based on the offending commit in order to minimize
future conflicts.

* commit '7e1864332fbc1b993659eab7974da9fe8bf8c128':
  riscv: fix race when vmap stack overflow