riscv: switch set_icache_stale_mask() to using non-atomic assign_cpu()

The atomic cpumask_assign_cpu() follows non-atomic cpumask_setall(),
which makes the whole operation non-atomic. Fix this by relaxing to
non-atomic __assign_cpu().

Fixes: 7c1e5b96 ("riscv: Disable preemption while handling PR_RISCV_CTX_SW_FENCEI_OFF")
Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>