io_uring: only test SQPOLL cpu after we've verified it

We currently call cpu_possible() even if we don't use the CPU. Move the
test under the SQ_AFF branch, which is the only place where we'll use
the value. Do the cpu_possible() test AFTER we've limited it to a max
of NR_CPUS. This avoids triggering the following warning:

WARNING: CPU: 1 PID: 7600 at include/linux/cpumask.h:121 cpu_max_bits_warn

if CONFIG_DEBUG_PER_CPU_MAPS is enabled.

While in there, also move the SQ thread idle period assignment inside
SETUP_SQPOLL, as we don't use it otherwise either.

Reported-by: syzbot+cd714a07c6de2bc34293@syzkaller.appspotmail.com
Fixes: 6c271ce2 ("io_uring: add submission polling")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2241,10 +2241,6 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
 	mmgrab(current->mm);
 	ctx->sqo_mm = current->mm;
 
-	ctx->sq_thread_idle = msecs_to_jiffies(p->sq_thread_idle);
-	if (!ctx->sq_thread_idle)
-		ctx->sq_thread_idle = HZ;
-
 	ret = -EINVAL;
 	if (!cpu_possible(p->sq_thread_cpu))
 		goto err;
@@ -2254,10 +2250,18 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
 		if (!capable(CAP_SYS_ADMIN))
 			goto err;
 
+		ctx->sq_thread_idle = msecs_to_jiffies(p->sq_thread_idle);
+		if (!ctx->sq_thread_idle)
+			ctx->sq_thread_idle = HZ;
+
 		if (p->flags & IORING_SETUP_SQ_AFF) {
 			int cpu;
 
 			cpu = array_index_nospec(p->sq_thread_cpu, NR_CPUS);
+			ret = -EINVAL;
+			if (!cpu_possible(p->sq_thread_cpu))
+				goto err;
+
 			ctx->sqo_thread = kthread_create_on_cpu(io_sq_thread,
 							ctx, cpu,
 							"io_uring-sq");