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: 7c1e5b9 ("riscv: Disable preemption while handling PR_RISCV_CTX_SW_FENCEI_OFF")
Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>

Author: YuryNorov

arch/riscv/mm/cacheflush.c

@@ -172,7 +172,7 @@ static void set_icache_stale_mask(void)
 	stale_cpu = cpumask_test_cpu(cpu, mask);
 
 	cpumask_setall(mask);
-	cpumask_assign_cpu(cpu, mask, stale_cpu);
+	__assign_cpu(cpu, mask, stale_cpu);
 	put_cpu();
 }
 #endif