RISC-V: Fix /proc/cpuinfo cpumask warning

Commit 78e5a33 ("cpumask: fix checking valid cpu range") has
started issuing warnings[*] when cpu indices equal to nr_cpu_ids - 1
are passed to cpumask_next* functions. seq_read_iter() and cpuinfo's
start and next seq operations implement a pattern like

  n = cpumask_next(n - 1, mask);
  show(n);
  while (1) {
      ++n;
      n = cpumask_next(n - 1, mask);
      if (n >= nr_cpu_ids)
          break;
      show(n);
  }

which will issue the warning when reading /proc/cpuinfo. Ensure no
warning is generated by validating the cpu index before calling
cpumask_next().

[*] Warnings will only appear with DEBUG_PER_CPU_MAPS enabled.

Signed-off-by: Andrew Jones <[email protected]>
Reviewed-by: Anup Patel <[email protected]>
Reviewed-by: Conor Dooley <[email protected]>
Tested-by: Conor Dooley <[email protected]>
Acked-by: Yury Norov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]/
Fixes: 78e5a33 ("cpumask: fix checking valid cpu range")
Cc: [email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: Andrew Jones

arch/riscv/kernel/cpu.c

@@ -213,6 +213,9 @@ static void print_mmu(struct seq_file *f)
 
 static void *c_start(struct seq_file *m, loff_t *pos)
 {
+	if (*pos == nr_cpu_ids)
+		return NULL;
+
 	*pos = cpumask_next(*pos - 1, cpu_online_mask);
 	if ((*pos) < nr_cpu_ids)
 		return (void *)(uintptr_t)(1 + *pos);